Black holes are one of the most fascinating objects in the universe. We still don't know everything about them, and what we do know is often corrected or revised, and scientists are constantly studying them and learning more about them.
Black holes are formed in one of two ways. One is that they form when massive stars die - "massive" meaning stars whose "birth masses" were 8-10 times the mass of our Sun. When these stars exhaust all their hydrogen, they explode and die, leaving behind a very compact, dense object - a black hole. This is a stellar mass black hole, whose mass is of an order of magnitude a few times the mass of the sun. The other way that black holes form is from the direct collapse of gas. This process is assumed to result in even more black holes - ranging from 1,000 to 100,000 times the mass of the Sun. This second method is believed to have happened in the formation of the early universe, and produces more massive black hole seeds.
Three types of black holes that are currently known to exist - stellar black holes, supermassive black holes and intermediate black holes (IMBH). Stellar black holes are formed when stars more than three times the mass of the Sun collapse into small but incredibly dense black holes. These black holes have an incredible amount of gravitational force that pull on objects surrounding it. Stellar black holes also consume the dust or gas from the galaxies surrounding them, which keeps them growing.
Supermassive black holes are millions or billions of times bigger than the Sun. These types of black holes are believed to be at the centre of every galaxy, including the Milky Way. It's not clear how these black holes are created, but once they are, they again feed on the dust and gas around them and grow even larger. Theories about the creation of supermassive black holes include the idea that they are formed from hundreds of smaller black holes merging together; the collapsing of large gas clouds; the collapse of a stellar cluster; or from large clusters of dark matter - something else that we don't currently know much about, but which we know exists because we can observe its gravitational effects on other objects.
IMBHs are a relatively new discovery, and are thought to form when star clusters collide in a chain reaction. In 2018, research on intermediate black holes showed that they might exist at the centre of dwarf galaxies.
As there is still much we have to learn about black holes, there are also many theories surrounding them that have yet to be proven or disproven. One such theory that has been proposed is the theory that we are in fact living in a black hole, proposed by physicist Nikodem Popławski. Popławski's theory is that the Big Bang was not the beginning of our universe, but in fact was the moment a black hole in another universe collapsed and created our own universe.
This theory is based on the Einstein-Cartan theory of gravity. This theory also extends general relativity to matter with "intrinsic angular momentum", or spin. It also includes such objects, concepts and principles such as affine connections, torsion tensions, and Hamilton's principle of stationary action - all of which, I shall be honest, I understand very little of as it's been quite some time since I took a Physics class. But basically, Popławski's theory is based on the idea of a "baby universe", which is a small, closed universe that has formed inside a black hole in another universe. When this black home collapses, the baby universe is released into space.
Proponents of Popławski's theory argue that the conditions we believe exist inside of a black hole are very hot and dense, which are very similar to the conditions that occurred during the Big Bang. Furthermore, we know that the universe is constantly expanding, which could be the result of the expansion of space inside a black hole. Followers of this theory therefore believe in the Big Bounce rather than the Big Bang, and this scenario also generates cosmic inflation, which seeks to explain why our universe currently appears to be spatially flat, homogenous and isotropic. If this theory is actually true, it would also explain the so-called "arrow of time", solve the black hole information paradox, and explain the nature of dark matter.
However, nearly all of the parts of this theory are just that - theories, guesses and conjectures. We have no way currently of confirming what actually goes on inside a black hole, as anything passing beyond the event horizon of a black hole, according to current theories, would either be "stretched out like spaghetti" or instantly burned to death, and so we cannot say whether or not the inside of a black hole could even support a universe. It's also important to remember that the Big Bang - or Big Bounce - are still just theories as well, and the creation of our universe could have happened in any number of ways.
There is no direct evidence that our universe is inside a black hole... but then again, there is no evidence that it isn't, either. We're still learning more about the universe and black holes every year, and the possibility is intriguing enough to remain a viable theory for many.
Sources:
Black Hole (Wikipedia)
Nikodem Popławski (Wikipedia)
"Black holes: Everything you need to know" (Space.com)