Why hydrogen reveals the temperature of dark matter  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.
[Via Starts With A Bang!. Image via HubbleSite]