Why aren't brown dwarfs the dark matter? There seems to be not enough matter to account for the fact that the speed of some stars located far from the Galaxy center is almost the same speed as those stars found nearer to the center of the Galaxy, therefore it seems there should be even more matter: so this missing matter is termed dark matter. There is a kind of stellar object that isn't massive enough to produce its own light but is many times more massive than Jupiter, a.k.a brown dwarf. Since it isn't bright enough to be detected by our deep space telescope nor the earth bound counterparts, they should be numerous in galaxies as I believe these objects sits in the middle range between planets and stars. However it seems the scientific community had ruled out this potential dark matter candidate without scanning the unobservable Milky Way galaxy as well as billions other with different light spectrums. Why aren't brown dwarfs considered candidates for the mysterious dark matter?
Two reasons.


We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the density of the universe can be in the form of normal matter (e.g atoms, protons, neutrons etc). As brown dwarfs are made of these things, they can only contribute to this small percentage and cannot be responsible for most of the dark matter (the same is true for anything made of normal, "baryonic matter").
Astronomers have counted how many brown dwarfs there are, both in the local neighbourhood and in star clusters (see for example Andersen et al. 2008;  Kirkpatrick et al. 2011; Burningham et al. 2013; . In other words we can see brown dwarfs - they emit most of their light in the infrared part of the spectrum, and surveys like SDSS, 2MASS and WISE have uncovered them in their thousands. It turns out that there is about 1 brown dwarf for every 4 more massive stars. So, although numerous, they contribute a very tiny fraction of the normal matter in our Galaxy. In addition, microlensing experiments towards the Galactic bulge and Magellanic clouds suggest that although there are lots of brown dwarfs elsewhere in our Galaxy (e.g. Alcock et al. 2000; Novati et al. 2008), there are not enough to make much of a contribution to dark matter, and so the results from our solar neighbourhood appear representative of the Galaxy as a whole.