How do I get rid of black gunk in my drains?

In all my drains in my house, there is this black slime nastiness accumulating on the inside. Is it mildew? I scrub the black off the inside of the drain plugs with a toothbrush and it comes up. Two weeks later when I go to clean it again, more black has ‘grown’ in. It also forms on the drain holes in my shower. My house is only 4 years old. Unfortunately, I cannot remove my drain plugs from my sink, at least I don’t know how.