“That’s all regular matter, just five percent. A quarter is “dark matter,” which is invisible and detectable only by gravitational pull, and a whopping 70 percent of the universe is made up of “dark energy,” described as a cosmic antigravity, as yet totally unknowable. It’s basically all mystery out there – all of it, with just this one sliver of knowable, livable, finite light and life.” -Summer Brennan
The Universe could have had any number of fates, even given that it started out with a hot Big Bang. Gravitation could have overcome the initial expansion, eventually causing a recollapse and a big crunch. The expansion could have been too great, and caused a runaway expansion that always slowed but never ceased. Yet the Universe gave us a surprising option that was none of the above: acceleration due to dark energy.
It’s our measurements of astrophysical objects at great distances and the fluctuations of the cosmic microwave background that allowed us to determine our expansion history. Yet those two sources, when we look at them in detail, give results that conflict with one another! Does this mean that one of the measurements is problematic? Or, perhaps more excitingly, does that mean dark matter or dark energy is changing over time?