Dark matter. Nobody knows what it is, but it's thought to make up a quarter of the universe. If that's so, theory predicts that thousands of dark matter clumps should surround the Milky Way, each holding a small satellite galaxy.
“But if we actually look out and try to find galaxies corresponding to these clumps, we only see about 10 to 20 of them there. This is a huge mismatch.” James Bullock of U.C. Irvine at a dark matter conference last week at the Space Telescope Science Institute in Baltimore.
To complicate matters, the biggest dark matter clumps are the best candidates to contain satellite galaxies. And they just don't have any. “The weird thing that we're finding is if we actually go out and measure the masses of the satellites that we can see, little satellite galaxies, dwarf galaxies that we can see, if we measure those masses, those masses are actually smaller than a good number of the dark matter clumps that we predict should be there.” [Michael Boylan-Kolchin, James Bullock and Manoj Kaplinghat, "Too Big to Fail? The Puzzling Darkness of Massive Milky Way Subhalos"]
It's not clear why some dark matter clumps won't form galaxies despite their ample size. Bullock and company have a name for those galaxy-free clumps: “We were calling them massive failures.” Here’s wishing the search for a reason is a success.
[The above text is an exact transcript of this podcast.]