How can Scientists be sure that Black Holes exist when you can't see them?

The readings that we get from the black holes influence on other matter (stars, entire galaxies in some cases) are the most telling signs that they exist. Black holes have so much mass and therefore so much gravitational pull that they actually bend light around them indicating a "void" of light. A black dot in space completely devoid of light. There are other ways to tell as well, stars in orbits that are gravitationally drawn to a certain area of space-time for no apparent reason. ie there is nothing there. Nothing, but a super-dense ball of matter that we can not see.