Most brown dwarfs are very cool: < 1000 K.
A blackbody that cool does not emit large amounts of light in the visible region of the electromagnetic spectrum. This makes brown dwarfs virtually invisible -- in visible light.
However, with the advent of sensitive infrared detectors astronomers have been able to find brown dwarfs in ever increasing quantities in the last decade.
Because brown dwarfs do not produce their own energy like stars --- they
are not fusing hydrogen into helium --- they cool indefinitly.
They are the cosmic equivalent of a hot rock. A hot rock that is heated
somehow (the Sun, a fire, etc.) will then cool off until it is in
equilibrium with (same temperature as) its surroundings.
Brown dwarfs are similar. They have heat left over from their gravitational collapse and then they quickly cool and fade.