Is there any connection between the two? And, why isn’t it called “Dark Gravity”, which it seems to be? Where does gravity come from? Why does it exist? What IS it?
Maybe it comes from another dimension and “Leaks” into ours. I dunno.
But dark matter and dark energy seems to me should be related. (Of course I don’t know why, I’ll leave that to others). 
The connection is the word “dark”.
Dark matter is really quite well established. We can see from galaxy rotation curves that the visible parts of galaxies must be surrounded by halos of invisible but massive matter (i.e. dark matter), and I believe there are instances where galaxies have interacted and we can see the effects of that halo in other ways.
Dark energy is more “we know the universal expansion is accelerating, so something must be pushing it, so let’s give that something a name”.
The next person will answer the “What is it” question. Subsequently, they will win the Nobel prize in Physics.
Substitute the word ‘unknown’ for ‘dark’ to break any sort of connection they may have.
But, since they’re both unknown… who knows, maybe they might be connected. It’s unknown and we’re in the dark.
Why not Dark Gravity?
What we call gravity is an effect and a force. It cannot have a detectable effect without being itself detected.
Where does gravity come from?
Basic physics here. Gravity comes from physical matter. If you wanna know more, read up on the Higgs Boson, the recently discovered carrier for gravity (just as light is carried by photons).
What is dark matter?
Dark matter is the hypothesized cause of gravity detected beyond what should be caused by the detectable matter (according to the current mathematical models). It’s not an actual THING, rather an idea.
Why does dark matter exist?
Basically because we say it does. It’s a plausible explanation for the gap between what our mathematical models say should happen and what is happening. Once we can explain that disparity, or detect the matter causing the additional gravity, dark matter will cease to exist.
Historically, the need for “dark matter” was determined from looking at the rotational speed of galaxies, around 1932*. The mass of visible matter, i.e. stars, was not enough to account for the mass that they knew galaxies had to have to be able to rotate as fast as they did. Dark matter includes every thing that isn’t stars, including normal matter like planets and dust clouds. Eventually it was realized that there wasn’t enough dark matter in the form of normal matter either. The meaning of dark matter has evolved to include, or even only mean, some new kind of particle that doesn’t interact with light.
Later, around 1998*, it was discovered that the expansion of the universe was accelerating, rather than slowing. This meant that there was some other force causing that. It was named “dark energy” analogously to dark matter. But there isn’t, as far as anyone knows, any connection between the two other than, as leahcim says, the word “dark”. If there were no term “dark matter”, it might have been called something else.
- New Scientist has a timeline up on its website right now that has the discovery of the need for dark matter (1932) and dark matter (1998).
I have propsed to label those phenomena atramentous corporeity and crepuscular palpitation, but unfortunately those terms have not caught on in the general cosmology community. Aside from both being placeholders for phenomena that are not well understood, ther is nothing connecting them, and resolving one will not necessarily give us insight into the other.
Stranger
Sort of correct and majorly incorrect, respectively. On the first point, physical matter is one thing gravity comes from, but not the only. There are some things that are only debatably “matter” which still cause gravity, and mass isn’t the only property of stuff which causes gravity. Gravity is produced by something called the stress-energy tensor, and energy density (which usually manifests as what we’d call mass) is only one of the sixteen components of that tensor. Notably, pressure (or tension, if it’s negative) accounts for three of the components, so if a substance were to exist whose pressure were of comparable size to its density, it would get most of its gravity from the pressure. This is believed to be the case of dark energy, and in fact, dark energy’s pressure is negative, meaning that its gravity is more repulsive than attractive.
On the second point, the (hypothetical, poorly-described, and not yet observed) carrier particle for gravity is the graviton, which is massless and spin-2. The Higgs boson, which is very massive and spin-0, has basically no particular relationship to gravity at all. You often hear that the Higgs is responsible for mass, but even that’s not quite true: It’s responsible for some mass, specifically the mass of most fundamental particles, but it is not responsible for, say, most of the mass of the proton or neutron.
And on one final point: We know with absolute confidence that at least some dark matter exists. Our entire planet and nearly everything on it, including us, is dark matter (after all, we’re not glowing). The catch is that so-called “baryonic matter” (i.e., the familiar sort of matter, that gets most of its mass from protons and neutrons) can only account for about 30%, at most, of the dark matter we deduce to exist in the Universe.
I have propsed to label those phenomena atramentous corporeity and crepuscular palpitation, but unfortunately those terms have not caught on in the general cosmology community. Aside from both being placeholders for phenomena that are not well understood, ther is nothing connecting them, and resolving one will not necessarily give us insight into the other.
As for where gravity comes from…ask your mother. Most physicists look at Misner, Thorne, and Wheeler’s Gravitation causing their bookshelf to bow in the middle and figure that it really is too pleasant a day not to go for a walk.
Seriously, gravitation is the presence of mass causing a warp in thr plenum of spacetime. Or quantum field interactions mediated by gravitons. Or little red men pulling downward. Take your pick; all explanations are equally probable, and fortunately, the power never goes on the blink, even when ConEd goes down.
Stranger
Very amusing aperçus, as usual, from our resident physicists. That we’re dark matter is cool; of course I never thought of it that way (ie, it is too blindingly obvious).
Not sure if I understand the statement that once we get our mathematical and other shit together dark matter will “disappear.” Means the metaphor becomes unnecessary (ie, just a good metaphor, dark==no light, dark==“unknown, shadowy reason,” ), or that some scientific realization will be made that photons are emitted (ie, wasn’t just metaphor)?
Also I never knew the fact that there is a “mighty 16” of check-list components of the stress-energy tensor. Given that I know, in the most idiotic of ways, what a tensor is, and what stress and energy are,* can you list the components so at least I can goggle at them and treat them as some sort of mantra?
*ETA: I re-read this and laughed out loud at what presumption permitted me to write it.
ETA 2: I see what I did there with “blindingly obvious.” Just came out that way, but hey.
Well, I would have said ask Chronos (which seriously was my first thought) but I figured you get asked this stuff enough and are only a few questions away from going postal. 
And thanks for explaining the baryonic vs. dark matter thing. I’d always thought dark matter was just regular matter we couldn’t detect for some reason (e.g. interstellar dust, non-reflective asteroids/moons/planetoids and the like).
Wikipediadoes have a basic breakdown of the components.
Like I said, some of it is. It’s just that the other stuff (i.e., most of it) is more interesting, because we don’t know what it is. So a lot of people, when they use the term “dark matter”, just mean the mysterious stuff.
“Dark matter” in general usage is actually referring to what cosmologists refer to as missing dark matter (MDM) or cold dark matter (CDM). Once we figure out the source, be it weakly interacting particles (likely), microscopic black holes (less likely), or adjustments to general relativity (highly unlikely) we will be able to replace the label with an actual hypothesis of the phenomenon. Right now, however, we know essentially nothing other than that there is a large percentage of total mass missing from our observations.
Stranger
You know, if science writers could just refer to “dark matter” and “dark energy” as “unknown matter” and “unknown energy”, it would be as helpful as if science writers went back to Leon Lederman’s original name of “that goddamn particle” instead of “God particle”. A billion wasted hours of bad thinking caused by getting the names spectacularly wrong.
At least the public understands that it doesn’t understand something called the Higgs boson and they don’t twist their brains thinking that the words mean something other than what they mean.
Unfortunate nomenclature is a wide problem in science in general. The supposed paradox posed by particle/wave duality is actually nothing but a failure to appreciate that in terms of quantum mechanics fundamental objects are neither a classical particle or wave. It doesn’t make for a paradox of any kind; you just have to treat the interactions of fundamental particles or quantum fields differentlt than you do raindrops or sound waves.
And to be fair to science writers, most open up with a clarifying discussion. You just have to read past the title. The yahoos who use Dawkins’ The Selfish Gene to justify perfidy have clearly not turned a single page in the book.
Stranger
Wow. Thanks all, for the responses! Very informative.
You guys and gals ROCK!
I’m not trying to be a snot, but that missing mass, when observed–even if (better) observed by its effects on other bodies but not in itself–will or will not be “dark,” ie not emitting photons?
Is that even possible with weakly interacting particles? I know that black holes are called black for a reason, yet I’ve seen discussions here–which I can follow when I read them but am not able to remember or say–of some sort of photonic trace or cloud or last-time hover. Or something.
Anyway, theres my question…
I recommend dstarfire’s wiki link, but if you want an even further reduction…
Unless you’re exploring new models beyond general relativity, the stress-energy tensor has certain symmetry properties that reduce the number of quantities needed to specify it down to ten. They are:
- energy density
- three components of momentum density
- six components of stress (three that are pressure and three that are shear stress)
It won’t be charged like protons and electrons, so in that sense it won’t be doing the things you usually think of when you think of making photons. That’s not to say dark matter can’t be made of particles that have electromagnetic interactions (and, thus, photon-producing interactions). The EM interactions would just have to come about in a sufficiently round-about way so as to keep particle interaction rates weak w.r.t. “normal” matter. On the other hand, they could be completely EM neutral.
There is a lot of interest at the moment in “complex” dark sectors. If dark matter comes from some unknown particle, there is no reason it can’t be due to multiple “dark” particles with their own “dark” interactions.