The Expansion of the Universe Accelerates

By studying how light travels through space, we've been able to find out something about the way the universe expands. And what happens then is general relativity. Once you turn big handle of all that mass, which is a bunch of fun, it'll tell you how the universe expands as a function of time. It's moving faster to day than it was yesterday, on average. If you then reverse the arrows, bacto, what then? The universe must be made of all this stuff.