system
Given the following post, respond as if you're commenting on Reddit:
human
Why do people tend to explain "dark matter" as some exotic form of matter never seen before?

We recently have found many brown dwarfs we didn't even know existed very close (in interstellar terms) to our own sun. Wouldn't it be more plausible that there are hundreds of billions or more of such objects, planets, comets, and other such massive objects that we just simply cannot see, because they do not glow? That would go a long way to explaining the much of the "missing mass" in the universe.
gpt
The "missing mass" on galaxy scales is not simply a matter of mass quantity, but is also a fundamental difference in mass **distribution**.

The Milky Way (as with most spiral galaxies) is made up of a central bulge and a flat disk. If normal matter was the only contribution to gravity, then stellar orbital velocities in the disk would behave the same way as planets in the Solar system; i.e. the further you are from the centre of mass (the galactic centre in this case), the slower you orbit.^* However, disk stars show no change in orbital velocity with radial distance (so-called 'flat rotation curves').

If dark matter was purely made up of undetectable regular matter (brown dwarfs in your question), then the total mass would be increased, but the mass would remain within that star's orbit. So, stars would orbit a little faster than expected from detectable mass, but orbital velocities would still vary with distance from the Galactic centre. 

In order to reproduce the observed orbital velocities for disk stars, the dark mass must be distributed fundamentally differently to the stellar matter. Specifically, dark mass density must drop off proportional to 1/r^2 and extend significantly further than observed mass. However, since brown dwarfs (and black holes etc.) have the same formation history as stars, they should be spatially distributed the same way. 

Thus, conventional (but undetected) matter cannot be the sole (or dominant) contributor to dark matter.

^* - This is the case when all mass is contained within the orbiting body's orbital radius. This approximation is realistic for stars a long way from the Galactic centre (like the Sun).