I have heard of something called HDM which is a theory to explain dark matter that is no longer as popular as it used to be. What is HDM?
Two theories to explain the composition of dark matter are Hot Dark Matter theory (HDM) and Cold Dark Matter theory (CDM). The main difference between the two theories is the speed of the candidate particles. As you may have guessed, HDM particles move quickly (and are thus "hot") while CDM particles move slowly. Neutrinos are the main HDM candidate for dark matter as they are very weakly interacting and exist in such large numbers in the universe.
The main problem with HDM theory is that the high speeds of the particles (i.e. neutrinos) in the early universe could not have allowed small density fluctuations to clump together in order to create the large fluctuations we see now. We believe matter (or in other words galaxies) is distributed throughout the universe as it is now due to the growth of small initial fluctuations. Since neutrinos would have been moving so fast that these tiny initial fluctuations would have been smoothed out, HDM theory cannot account for the distribution of galaxies in the universe. The small scale of this initial clumping that is impossible for neutrinos to maintain is supported by COBE observations.
Now when HDM is discussed, it is usually in combination with CDM (the combination is called MDM or "mixed dark matter"). HDM is thought to be limited to at most a few percent of dark matter if the amount is even measurable.
This page was last updated June 27, 2015.