What is a Black Hole?

Black Holes are objects in space where gravity is so strong, not even light can escape.
According to NASA, the first black holes appeared at the beginning of the Universe, though it is generally accepted that most black holes in the Universe today are a result of the collapsing cores of massive stars. When a massive giant star burns through it’s hydrogen fuel it starts creating helium. Helium is then transformed into heavier elements such as carbon (this for instance will be the end of the road for our Sun). Due to the massive gravity of the giant star (at least 20-30 times larger than our Sun), carbon continues fusing into heavier elements such as iron. When this happens, the star has seconds left to live. It is no longer able to support itself, thus exploding into a Supernova, while its core collapses into a neutron star or a black hole. Due to the immense gravity of black holes, they start eating up everything in their path (planets, stars, gas, asteroids etc.) even smaller black holes. Black holes can come in very different sizes. Some are infinitesimal, others are supermassive, like Sagittarius A*, the black hole at the center of our galaxy. However, there are still a lot of scientists today that argue that black holes don’t actually exist. Truth is we cannot see the black hole itself, because it doesn’t emit light, but we can see the accretion disc around it, a region where matter swirls around the black hole at such high velocities that it heats up at incredible temperatures, thus emitting light.

NOTE: This post is merely the tip of the ABC about black holes. These objects are so complicated and theoretical, you need to read entire books to fully understand the concept of black holes (which is mostly theoretical). Stephen Hawking has a series of books about black holes, such as: A Brief History of Time: From the Big Bang to Black Holes, Black Holes: The Reith Lectures, The Big Bang and Black Holes, etc. – Roman Alexander

(The question was originally asked by Herbert Omolaso)