From Nature:
A computer model has made progress in solving an astronomical mystery: why is so much hydrogen in the Universe paired up into molecules instead of existing as single atoms? The secret is simple. It comes down to the fact that space dust is probably bumpy rather than smooth. It has long been assumed that hydrogen atoms sticking to these dust particles are jostled together, encouraging hydrogen atoms to pair up into H2. But when one team of researchers tested this theory, it came up short.
More here.