Brown Dwarfs; From a stellar formation graph and formula page:
These so- called "aborted stars" have a mass around .08Ms, and can't convert hydrogen into helium (Their cores never reach the threshold temperature for hydrogen burning.). The only energy radiated is due to gravitational contraction (Kelvin-Helmholtz contraction), which is why they are difficult to detect unless they are located near us. To determine if brown dwarfs contribute substantially to the dark matter mass, we need to estimate their number. We can use the Stellar Mass Function, F(M) (proportional to M-2.33), to extrapolate the number of brown dwarfs from the numbers of more massive stars. If F(M)dM = the number of stars with mass between M and M+dM, then the total mass contribution from brown dwarfs can be written as (M)(F(M))dM. The question is, how does F(M) behave for very low M? F(M) is proportional to M-2.33 only for main sequence stars, not for brown dwarfs. In general, giant clouds of gas and dust collapse, then fragment. The smaller fragments, which become K and M stars, are more abundant then the larger fragments, which become O and B stars. However, the amount of small fragments drops off right near the size needed to make brown dwarfs, limiting the number of brown dwarfs that could exist, and also providing an uncertainty as to that number.
If all the dark matter is composed of brown dwarfs, we would need one brown dwarf every 30 cubic ly of space, many trillions over the entire Milky Way. The number of known brown dwarfs and brown dwarf candidates in our section of the galaxy are exceedingly slim, however, making it unlikely that the density of the galaxy's brown dwarf population accounts for significant dark matter.
(Some original, some stolen text)