I'm working on some other posts right now, but I wanted to make others aware of some more recent evidence that dark matter could be 7.1 keV sterile neutrinos.
(Update July 5th 2015: Note that I've written an update to this post since the 3.55 keV emission was not detected with any significance within dwarf spheroidal galaxies...which are high in dark matter and low in baryon-sources of X-rays. Though, I've keep the rest of the article below intact for record keeping.)
First, Boyarsky et al. measured a signal at 3.55 keV within the Center of the Milky Way Galaxy that their model couldn't explain. The signal that they measured is much wider than can easily be explained by emission from Argon, Potassium or Chlorine ions. It's clear that this signal is not instrument noise because it doesn't show up in a blank sky scan and because it shows up at different values in galaxies with different z values (i.e. it's being redshifted when it comes from galaxies farther away from us.) Also, it's pretty clear that the signal is related to dark matter because the flux increases roughly linearly with the dark matter content of the galaxy.
A 7.1 keV mostly-sterile neutrino particle is an interesting dark matter particle because this rest mass falls right in the middle of the ~2-10 keV range of rest masses that is consistent with both data on Dark Matter Halos and Lyman Alpha Forest. (See post on The Case for keV Dark Matter.)
I also want to point out that this 7.1 keV sterile neutrino is not ruled out by cosmological data. For example, there is a recent paper by Vincent et al. that puts constraints on the mass&mixing angle for sterile neutrinos if they make up 100% of the dark matter (see Figure 2.) However, at 7.1 keV, the constrain is well above the value of mixing angle as measured by Boyarsky et al. and Bulbul et al..
There is also another reason to be interested by a keV mass sterile neutrino (however, it should be noted that the details below are completely speculative):
Let's imagine that a keV sterile neutrino could somehow decay into many, many light active neutrinos. (We can rule out the mechanism above in which 1 sterile neutrino decays into 1 light neutrino and 1 photon of half the energy of the sterile neutrino's rest mass because we need the sterile neutrino to decay into millions of active neutrinos.) However, knowing that a sterile neutrino can decay to a neutrino means that it is somewhat active in the weak nuclear force. This means that, if a black hole could consume sterile neutrinos, then it's possible that a black hole could eat dark matter (and regular matter) and spit out millions to billions of active neutrinos. We know that light active neutrinos can be ejected from the event horizon of a black hole. As such, it's possible that one heavy sterile neutrino of rest mass of ~keV could turn into billion of active neutrinos of roughly micro-eV rest mass (provided that these active neutrinos have virtually no kinetic energy.) Also, if black holes can consume Fermi degenerate sterile neutrino dark matter, then this can provide a mechanism that allows super-massive black holes to form (and not run-away in size because the density of keV sterile neutrinos is limited in the center of galaxies because they would be limited by Fermi pressure.) This would help to explain the major question of how super massive black holes formed but did not consume their entire galaxy's dark matter. GeV cold dark matter would not solve the super massive black hole problem in physics.
If there are really ~7 million times more active light neutrinos today than the expected number of ~60 per cm3, then this is just the number of light active neutrinos required to provide a quantum degeneracy pressure of 7∙10-30
g/cm3. The density of light neutrinos would need to be 420,000,000 per cm3 in order to reach this Fermi pressure, and this in turn would require a lot of matter and dark matter converting into light active neutrinos within super massive black holes. Well, this turns out to be nearly exactly the same energy density as dark energy today (7∙10-30 g/cm3). In other words, if the actual density of nuetrinos is roughly 420 million per cm3 rather than 60 per cm3, then we could explain dark energy as the Fermi pressure supplied by quantum degenerate light neutrinos.
This means that, while highly improbably, it might be possible that dark energy is the quantum degeneracy pressure of light active neutrinos that have decayed from keV mostly-sterile neutrinos. This last part of about neutrinos as dark energy is still highly speculative; however, the information above about 7.1 keV sterile neutrino dark matter is starting to firm up. We'll have to await the launch of the Astro-H satellite in order to nail down whether the 3.55 keV emission signal is actually from the decay of sterile neutrinos.