Our universe is not only expanding – but that expansion is accelerating. It’s expanding faster and faster and faster. When you’re driving your car you don’t speed up for no reason – either you step on the gas, you’re rolling down a hill, you get slammed from behind, or your engine suddenly and unexpectedly starts feeding gas to the engine (I hope not!). The universe’s expansion can’t speed up for no reason either. That energy MUST come from somewhere – but we don’t know where, so we’re calling this extra energy “Dark Energy.”
Law of the Universe:
You cannot create energy from nothing. Just like you wouldn’t expect to see a kitten materialize out of nothingness and appear on your keyboard (it might jump there, but not materialize!), you can’t get energy out of nothing. It all comes from somewhere. This is called the law of conservation of energy. Sometimes it’s called the first law of thermodynamics.
The Universe is Accelerating:
From our measurements of Type 1a supernovae we know that the universe is expanding and that expansion is accelerating. Basically, those supernovae are farther away than our physics models predict them to be, and there’s not enough energy in the Universe to do that.
Einstein proposed a “cosmological constant” to solve a similar problem many years ago. Then he changed his mind and gave it up. With our new observations, it’s back in play now. Under this hypothesis, a specific volume of space (the cosmos) always (constantly) has a given amount of energy in it. Say you had one cubic meter of space. Under this theory, it would have … let’s say an amount “5” of energy. Then, over time the universe expands and now, your one cubic meter of space now takes up two cubic meters. Because of the cosmological constant, that space would have an amount “10” of energy. By definition. Just because it is space. (Usually we do this with variables in astronomy, but I wanted to be clear). Pretty weird, but you get lots of extra energy.
What if Dark Energy is not constant? What if it changes over space and time? You’ve got a little more here, a little less there. There was more last year than there will be in a billion years. That’s the quintessence hypothesis. Maybe it’s particles (like photons and electrons), or maybe it’s a field (like the magnetic field of the Earth), but in any case it changes, it’s not constant.
And, not to be forgotten, what if our equations for Physics are just wrong? What if we need to completely rethink them? In any case, the universe is accelerating in its expansion, so until we know what is making it do that, we’re just going to call it Dark Energy.