Question #1: If black holes are "black," how do scientists know that they are there?
Well, as a general rule, a black hole cannot be seen because of the strong gravity that is pulling all of the light into the black hole's center.
However, scientists can see the effects of its strong gravity on the stars and gases around it. If a star is orbiting a certain point in space, scientists can study the star's motion to find out if it is orbiting a black hole.
When a black hole and a star are orbiting close together, high-energy light is produced. Scientific instruments can see this high-energy light.
A black hole's gravity can sometimes be strong enough to pull off the outer gases of the star and grow a disk around itself called the accretion disk. As gas from the accretion disk spirals into the black hole, the gas heats to very high temperatures and releases X-ray light in all directions. NASA telescopes measure the X-ray light. Astronomers use this information to learn more about the properties of a black hole.
Ever since Albert Einstein reluctantly predicted the existence of black holes with his theory of relativity in 1916, we’ve been making further predictions and gathering evidence. In 1935, physicist Subrahmanyan Chandrasekhar hypothesized that a star might become so massive it would collapse under its own gravity. Then in 1969, Donald Lynden-Bell suggested that supermassive black holes at the center of galaxies could be responsible for the huge energy signatures detected there, far beyond what stars alone could generate.
In 2015, very definitively, we “heard” for the first time the sound of two black holes colliding, a billion light-years away, with the detection of “gravitational waves” by the LIGOs (an event known as “GW20150914”) providing further supporting evidence to Einstein’s theory.
Question #2: What did we actually “see” on 10 April 2019?
We can’t pick up a telescope, even one the size of the Earth, and see the black hole for ourselves. The image that was shown to all on 10 April was constructed from the radio waves collected by a number of telescopes that are then assembled into the picture and translated into color for our benefit.
The Event Horizon Telescope (EHT), a collaboration of 8 radio telescopes around the world, pieced together this picture from observations made from Antarctica and Arizona. The black hole in question is the Messier 87, which is 55 million-light-years from Earth and 6.5 billion time the mass of our Sun. (The Sagittarius A*, the black hole at the center of our own galaxy, is another target of the EHT).
Because no single telescope is powerful enough to distinguish a detail as relatively small as a 55 million-light-year-distant black hole, scientists harnessed these 8 observatories together to simulate a much larger instrument, one as large as the Earth itself. It took 2 years and more than 200 people sifting and refining the data gathered on four days in April 2017 to bring the final famous image into focus.