Because they are composed of different kinds of antimatter, black holes are only visible when they absorb gases and other molecules from a given galaxy. The energy contained within the black hole ignites the gas, making the form of the black hole visible to scientists in differing wavelengths.
Black hole mergers are an astrological phenomena that signal a merging of two galaxies. The collision of two black holes is said to make all of outer space "jiggle," as waves of gravity ripple out from the impact across the universe. The collision is caused by the gravitational pull of one black hole attracting another, and the mass of the "smaller" hole is absorbed by the larger black hole as a result of the collision.
Detecting Black Holes
Waltzing Black Holes
Each galaxy contains at least one "supermassive" black hole that contains a mass of at least one million times that of the sun. Because these supermassive black holes have the greatest gravitational pull of any celestial body, they are said to be "waltzing" across their various galaxies due to the pull of other black holes in galaxies of relative proximity.
Black Hole Mergers
Traveling at over 200 kilometers per second, black holes move relative to waves of gravitational pull from other black holes or dying suns. Until they begin their waltz, the distance between each black hole relative to the other is about 4 to 7 billion light-years apart. The merger can take millions of Earth years, but the result is the formation of a new larger black hole.
Supermassive Black Holes
Supermassive black holes are the result of a black hole mergers. Their increased gravitational pull, the result of the collision, determines their particular galaxy's properties. New galaxies are formed as the result of two galaxies colliding, and the pull of the black hole within each determines which one will supersede the other in terms of blending the elements contained within each. Scientists describe this phenomena as the "co-evolution" of host galaxies with the black holes inside.