It is now believed that at the center of each galaxy there is a super-massive black hole that is millions to billions of times heavier than our sun. The massive black hole captures nearby stars and drags them into a swirling accretion disk. (Goddard Space Flight Center)

There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. (Black holes FAQ)

Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. (Wikipedia)