Here are my personal thoughts in a nutshell about the reason for the existance of the Universe. Note that I am not a professional scientist, philosopher, or theologist, just a (fairly intelligent) guy who has read a lot of books.
What process causes the physical existence of the Universe? This question can be divided into two parts: Why is there physical existence rather than nonexistence, and why this particular existence out of all the possibilities? The ultimate cause of physical existence, or CoPE for short, must also be self-contained, meaning that no outside agent is allowed (since any outside agent must itself be explained, leading to an infinite ladder).
The only candidate for a CoPE meeting the above conditions that I know of is the following: The universe is in fact a mathematical object that appears to physically exist from ‘inside’, but that exists only in potential from the ‘outside’. A more detailed explanation of this idea follows:
The Universe is widely accepted to follow certain mathematical laws. More accurately, the Universe has been observed to always behave in ways that can be described by mathematical equations. A reasonable extension of this observation is that every aspect of the Universe is described by mathematics.
A crude simulation of the Universe is not the same as the Universe. As the simulation becomes more accurate, it describes the Universe more accurately, but it is still not the Universe. However, a perfect simulation of the Universe is another matter. A perfect simulation of an element of the Universe would feel the same effects, and respond in the same way, as the ‘actual’ element of the physical Universe. A perfect simulation of the Universe would be just as real, viewed from ‘within’ the simulation, as the actual Universe. By that I mean that there would be no way to tell the difference, from within the Universe, between physical actuality and a perfect simulation of physical actuality.
Does this simulation have to actually be running on some gigantic supercomputer? No. All that would be needed are the mathematical laws underlying the simulation, and the initial state of the simulation. All the evolution of the simulation is already present in potential from that information. What running the simulation would do is allow the evolution of the internal state of the simulation to become visible ‘outside’ the simulation, but the experience of the ‘inside’ of the simulation would be the same regardless of how many times the simulation is run: one million times, one time, or never. An analogy would be an algorithm that produces the prime numbers. Running it would allow an outside observer to record the prime numbers, but the prime numbers would still exist even if the algorithm is never run.
Combining the above three paragraphs, even a potential perfect simulation of the Universe would be identical, when viewed from the ‘inside’, with the actual physical Universe, even if it is never actally run. So there is no need for an ‘actual’ Universe outside of the simulation at all!
Instead I propose that all that exists is mathematics: every possible logically consistent mathematical system exists, and each such system is a potential simulation of a particular Universe. In some of these simulations there exist self-aware structures that perceive the mathematics that they are part of as having physical existence.
There you have it. Flame away.