@'zilla , if I could pick your brain on that one. Is dark energy something that's been proven as an energy, or simply a deus ex machina inferred from the accelaration of the universe's expansion? Like, "The universe is accelarating! I must need energy to do that! Let's call it dark energy."
Also, is dark energy the same as dark matter? Perhaps the latter is being transferred into the former.
I could go and find these answers for myself... but then, as you're here
If you could really find those answers, you'd get the nobel price for sure
Is it an energy? Yes, unless General Relativity is incorrect. There are two major effects of dark energy. One is the expansion of space, the other is the very shape of spacetime. Einstein proposed that the shape of spacetime is intimately linked with the energy contained within it. We've measured the shape of spacetime on the largest scales, and it requires a lot of invisible energy to explain it.
In itself, this energy has a positive gravitational effect. It's the pressure of dark energy that overcomes this and drives the expansion. Exactly how these two effects are linked, still puzzles me. This guy seems to understand it.
Are dark energy and dark matter related? Unlikely, but not impossible. Whereas dark energy provides antigravity, dark matter provides the gravity necessary to form galaxies etc. We can create a map of dark matter, whereas dark energy seems smoothly distributed across the 'verse. Still, some scientists are following an idea called Dark Fluid, which is a combination of the two. Just like you propose, they imagine that dark energy can condense into dark matter. I think it's really exciting but I haven't gotten around to read about it yet.
Our broken universe
This is something I'm reading up on at the moment. It's called the electroweak epoch. It starts way way back when the universe was less than a second old, and when the universe got stuck in a rut.
Everywhere was really hot back then. I mean millions of billions of degrees. Everything was wild and, perhaps, simpler than the universe we know today. There were fewer forces of nature, because they kind of melt together at such high temperatures.
One of these forces was the electroweak force. Unlike today's electromagnetic force, which rules electricity and magnets and creates light, the electroweak force had four different particles similar to the particles of light (the photons). Otherwise it was similar to the electromagnetic force – it could attract things and repel things. But it must have had some other weird effects too. I don't know the details yet.
Starting in the electroweak epoch, however, and as the universe cooled down, something happened. The electroweak force was doomed. Why?
I can try to explain it with a comparison. Imagine biking really fast along a road. The road has huge, bowl-shaped potholes, but you're going so fast you're just soaring over them without falling down. The moment you go too slow, though, you will fall into a pothole. If you cannot bike faster again, you will always be stuck in that pothole, going back and forth along its sides. That is what happened in the early universe. When the temperature cooled down, the typical energy was small enough for part of the universe to fall into a pothole.
This did something profound, which was to break the electroweak force into two very different forces. The four electroweak particles were mixed together and partitioned out. Three were given to the weak force, and one was given to the electromagnetic force – it became the photon.
What does the weak force do? It is the weirdest force. It violates a whole bunch of rules that all the other forces respect. It is the source of radioactivity. It has strange ties to electromagnetism. It might have something to do with dark matter. And it can only be felt at a tiny distance which is less than the size of a proton. Why?
Because of that pothole. A very strange pact was made when the universe fell down there. With whom? The Higgs field, that enigmatic source of the Higgs bosons that we discovered in 2012! In fact, ever since the end of the electroweak epoch, the weak force has had close ties with this field, while the electromagnetic force goes free. Because of the Higgs field's grasp on the weak force, the three particles of the weak force are always eaten by the Higgs field, before they can get anywhere at all. And it doesn't end there. The weak force is thought to act as the messenger between the Higgs field and most of the other particles, too!
What does that really mean? It happens to give all those particles a mass. Of all the particles we have observed, only the photon is without any mass. That enables it to always travel at the speed of light. So... if it wasn't for this pothole, you could argue that everything would be moving at the speed of light! And you know what happens when you travel at the speed of light? Time does not pass.
Weeell.. actually, there is reason to believe that it wouldn't be quite like that. But it surely wouldn't be the universe we know, the universe we are able to exist in. So, it is actually quite a good thing that we live in a universe with a leg stuck in a pothole.
Interesting correction: when I say that the universe fell into a hole/was broken, it's not really as if a global change happened instantly. Even if it was perfectly synced, there shouldn't be anything stopping us from achieving those energies again, apart from the fact that a million billion degrees is a lot! So even before the universe got cold enough, that pothole was already there, waiting..