Question

Can a black dwarf be seen? Why or why not?

Answer 1

Not directly, for two basic reasons:
(1) They emit no significant radiation (light, heat, x-rays, etc).
(2) There are no black dwarfs yet.

Explanation:

A black dwarf is a white dwarf that has cooled sufficiently that it no longer emits significant radiation. It is estimated that it takes about `10^15` years for this to happen. The age of the universe is approximately `13.8` billion years, i.e. `1.38 xx 10^10` years. So no white dwarf has yet had enough time to become a black dwarf.

In theory a black dwarf should be observable by its gravitational effects on nearby objects.