Hydrogen is the simplest of all atoms, and it's easily the most abundant, accounting for 90% of all atoms in the universe. And this fundamental building block may have something to tell us about one of the universe's great mysteries.
That's what theoretical astrophysicist Ethan Siegel argues over at his ScienceBlogs site Starts with a Bang! He explains that, because we can see the radiation emitted by hydrogen atoms throughout the universe as they interact with other particles and jump energy levels, and because this allows us to see how the structure of the universe over time, this actually allows us to deduce the temperature of dark matter! It's a simple but awesome bit of logic, and I heartily recommend you check out his post to see all the steps in between and his final conclusion, all of which touches on a lot of what we both do and don't yet know about astrophysics.