Despite its invisible interior, a black hole can be observed through its interaction with other matter.
A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.
Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies.
After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.
Black holes can cause the following:
Death of the planet
Although it is believed that there is super-massive black hole in the center of our galaxy, we are on one of the spiral arms, far from the hole.
If the hole got close to Earth, there would be disruption.
Even from a billion miles away from our solar system, a black hole could cause a disruption in Earth's orbit, leading to changes in our tide.
If it got really close and crept up next to Earth, the resulting havoc would cause earth to career out of its orbit, spinning out of the solar system, or in the opposite direction, toward the sun, ending all life on it.
Stay out of it's event horizon, a radius from which there is no return.