I’m certain this question has been asked before and I’m just not smart enough to search for it, but: if matter and energy can neither be created or destroyed, where did it all come from in the first place? A friend of mine was getting all existential horror-y and I was trying to find an answer to satisfy them.
The correct answer is, we don’t know.
Oh, that’s not gonna help the freaking out at all. 
Not only do we not know, it is likely we will never know. At best, we can just guess at what conditions were like at the moment that matter and energy (which are two aspects of the same thing) came to be.
Stranger
I was taught that all of the matter and energy in the Universe was once crammed into a single point. Then, at some point, the Big Bang happened. How that small point got there is anyone’s guess.
A wizard did it.
Indeed, we don’t know. But just a bit of a nitpick – the idea that “matter and energy can neither be created or destroyed” is a Newtonian concept out of classical physics that isn’t valid in the quantum realm, where matter and energy are the same thing and completely interchangeable. In the very early universe, there was no matter as we know it, and the four fundamental forces (the strong and weak nuclear forces, gravity, and electromagnetism) were all unified. It was only as the universe evolved that the first quarks formed, then subatomic particles, and eventually atoms and the familiar constituents of matter, but only the light elements. Much later, the first stars became the factories for the creation of the heavier elements that eventually made life possible.
There are places to start learning about physicists’ best current ideas. One of them is called vacuum energy.
Vacuum energy is an underlying background energy that exists in space throughout the entire Universe.
This solves a lot of the philosophical problems. If there is always something and never nothing, and quantum mechanics says that stuff can, for a lack of a better word, congeal out of vacuum energy, then the answer to where did it all come from pops out as well.
As always, nothing involving QM is this easy. Vacuum energy surely exists. That doesn’t necessarily imply that the universe was created from vacuum energy or even that the question has any meaning in our current understanding. How much vacuum energy exists is also an open issue. And when you throw general relativity into the pot, all bets are off.
Still, you might want to find a beginner’s explanation of vacuum energy because it’s fascinating all on its own, and its implications touch just about everything in cosmology. That might give you a starting place to extend tendrils to lots of other good stuff.
Initially there was nothing (we think), then somehow nothing got torn in half and now there is something.
I’ve always enjoyed the idea that Entropy, the ‘disorder’ of a state of existence favors complexity. The physics that favors a splattered egg on the kitchen floor also means that eggs made of complex proteins and civilizations that make kitchen floors are possible, and indeed their existence.
Under this logic, the ultrahot soup of particles that formed immediately after the Big Bang weren’t capable of even atomic sized structures; indeed, even in Particle Colliders scientists have discovered the high-energy union of the Electromagnetic and Weak Forces. As the universe got bigger, it got colder, and it got a lot more options to start building new atoms and molecules.
We also know that energy and matter are intrinsically related. Energy released in chemical and nuclear reactions actually slightly reduces the mass of the components, and the antimatter-matter reaction directly converts mass into energy. For unknown reasons antimatter was the weakest link and said goodbye. However, large amounts of matter and the current acceleration of distance parts of the universe suggest that we have not even well accounted for matter and energy in detail.
For all we know, however, the Universe may have been a particularly advanced egg, dropped into a Kitchen Floor, and all evidence of that act itself is now gone.
Now that’s an interesting creation myth: “And God dropped an egg on his kitchen floor, and breaking of it was called the Big Bang.”
You aren’t the first to think of it, though. 
To nitpick the nitpick: conservation of energy is perfectly valid in the quantum realm; there are, of course, processes which change one type of particle into another, including processes such as pair production where a photon produces an electron and a positron, but those themselves are subject to strict conservation laws. Mass and energy are often said to be ‘equivalent’ in special relativity (and hence, in special relativistic quantum field theory, which underlies these particle conversion processes), but it’s probably more useful to think of mass as a type of energy, and of rest mass as that amount of energy in a system that can’t be transformed away by going to a different frame of reference.
In general relativity, the conservation of energy is a more delicate idea, since it can only be locally valid; indeed, there are prominent voices that argue we should call a spade a spade and just acknowledge that energy is not conserved.
Things are a bit more subtle than that. First of all, of course, no philosophical problems are solved in this way: we may get a way to create a universe from the quantum vacuum, but that just poses the question, but why the quantum vacuum? The quantum vacuum is far from nothing, and substituting one for the other is just rhetorical sleight of hand. (Which doesn’t stop people from selling books on this basis, but when has it ever.)
As for the existence of vacuum energy, if it did ‘exist’ in a naive way, then we wouldn’t: there’s a finite minimum energy to every quantum system that can be approximated by a harmonic oscillator, and a quantum field consists formally of an infinite number of these oscillators, leading to an infinite contribution to the vacuum energy. But even if we assume that, if we get right down to the smallest length scales, there’s some cutoff (typically assumed to be the Planck length), we get a contribution to the vaccum energy that’s 120 orders of magnitude greater than what’s experimentally allowed for (in the form of a cosmological constant)—a force large enough to rip everything apart into its subatomic constituents. So if vacuum energy is a physically meaningful notion, it’s not quite what we think it is.
As for ‘things spontaneously popping into existence’… Well, I think that’s a bit of a dicey concept, as well. It depends on an interpretation of the energy-time uncertainty relation that says, you can ‘borrow’ some energy as long as you give it back soon enough, but that’s not necessarily a good interpretation. The intuitive picture comes from so-called Feynman diagrams: the little squiggly pictures everyone’s familiar with if they’ve ever seen a PBS physics special. They look a lot like particles traveling through space, and they sometimes include loops and the like, where it looks like particles are being created, and then destroyed; and for these ‘internal’ lines, it’s indeed the case that the usual conservation laws don’t hold.
The only problem is that no Feynman diagram corresponds to a physical process; rather, each physical process is associated with an infinite series of Feynman diagrams, and only the complete series fully describes the process. So they should best be thought of as calculational tools, which can be used to compute the probability of some particular outcome of an interaction, given what went into it. So I think one should be cautious about relying on this picture too much.
Now, there are ways in which quantum mechanics can elucidate the creation of matter. One are tunneling scenarios, another are things like the no-boundary proposal of Hartle and Hawking, and so on. But they don’t really get to answering, well, but why this? Why anything? And that’s a metaphysical question, not a scientific one. That doesn’t mean we’ll never find an answer, but as of right now, it’s difficult to even say what an acceptable answer might look like.
As a final nitpick, maximally entropic states aren’t complex, typically. Picture milk dispersing in coffee: you start out with a low-complexity, low-entropy state of well-separated milk and coffee, then mixing starts, which leads a higher-entropy, high-complexity state of eddies and swirls, and at some point, you’ll have a uniform mixture—which is maximally entropic, but of low complexity.
PBS Space Time has many relatively easy-to-understand quick videos on this question, including playlists that build up to the OP’s question. Here’s one:
True, but not really relevant, since the OP can then be phrased as: Where did the interchangeable stuff come from?
I’ve gotten the impression that the better way to phrase it is that the sum total of all the matter and energy and antimatter and antienergy cannot change. Thus it is perfectly okay for some matter to pop into existence, provided that it is accompanied by a corresponding amount of antimatter. Does that answer the OP?
I couldn’t resist:
In the beginning, there was [insert deity of choice] …
This makes no attempt to explain where they came from but hey ho…
As I said. The perpetual problem on the Dope is that when physicists start talking subtle they almost certainly and instantly lose the naive audience that is asking the questions. Naive meaning anybody is who not conversant with post-graduate level physics.
Yes, there indeed are books written by actual physicists for the lay population on this topic. Whether the books can fully ensure that the readers learn both the gross and the subtle points I can’t say. But, as books, they can start much farther back and build up to the concepts that are subtle, providing much excellent physics along the way. The ones I’ve read all include the caveat that we don’t know much if anything for sure, but their totality is asymptotically better than just “we don’t know.” Additionally, as a lay person it’s difficult for me to appreciate whether “we don’t know” is a universal or it’s a stance taken by a segment of the community, with certain small things widely agreed upon.
I’ll stick with my recommendation for anyone interested, even without a physics education, to find some good beginner’s books or online materials and work through them. Most popular science books keep equations to a minimum (removing all subtlety, true) and use plenty of metaphors and examples (loose and tricky as it may be to explain quantum properties with classical materials).
Maybe readers won’t be conversant in all the latest scientific nuances, but in MHO the effort rewards more than “we don’t know” as an answer.
Matter is an illusion. It is all energy. Just some of the energy clumps together.
Exactly! It is wrong to talk about matter and energy as if we are talking about two different things, they are two forms or states, of the same thing.
^^Everything is made of that one thing, therefore every thing is the same as every other thing. But maybe sometimes we should refer to things in different forms and states as different things just to break up the monotony.
An equally valid answer, in comparison to all the talk about Vacuum energy and Newtonian physics, can be found in the Wikipedia entry for Hindu Cosmology from 10,000 years ago. Its a valid theory too, n the absence of any credible theories from present age. Its the first theory written by Hindu theorists, godmen call them what you will. They were the first to talk about the scale that we now accept. In a nutshell it says, the universe has existed for millions of years. It says that the universe follows a cycle of millions of years in existence and then disappears for million bs of years only to reappear for millions of years. And this cycle has been repeated for millions of years. As good as any theory.