They call them brown dwarfs because they’re small and don’t give off nearly as much light as the Sun. Thus, they are difficult to see. Even with our largest telescopes, it could be easy to miss a nearby brown dwarf. But they are made of the same stuff as the Sun: hydrogen, helium, etc.; i.e., not dark matter.
Brown dwarfs lack mass to start hydrogen-hydrogen fusion. I think the threshold is somewhere between 0.05 and 0.08 solar masses. Brown dwarfs still give off heat due to gravitational contraction, much like Jupiter and other large gas giants.
Some brown dwarfs are speculated to have fusion reactions taking place, however these would involve deuterium or lithium. The “pressure” threshold to get these started are considerably lower than for H-H fusion. The burn however would be short (on a cosmic timescale), since the object would quickly run out of fuel.