| Initially considered a glitch in the equations of Albert Einstein's theory of gravity, black holes were proved possible in the 1970s. Since then, astrophysicists have located loads of real black holes in the universe by detecting their light and X-rays and the reverberations they set off when they collide. But despite decades of study, physicists are still struggling to unravel the ways in which black holes sculpt — and are sculpted by — the cosmos. And they still don't know what these dense dollops of mass really are, deep down.
That's because black holes are essentially a pure manifestation of the gravitational force, and physicists don't know exactly how gravity works. Each theory of gravity comes with a distinct picture of a black hole — some wildly different from the standard description. That's why physicists tend to fixate on black holes. The mysterious objects act as a playground in which to explore the real-world consequences of our gravitational speculations.
What's New and Noteworthy One undeniable truth about black holes is that they're not made of matter, of atoms you can find on the periodic table. A black hole is what you get when you pack so much stuff into a region of space that it all collapses. The region takes on a dramatic new identity — one that depends on your preferred theory of gravity.
The classic picture of a black hole is the one that comes from Einstein's general theory of relativity, the most established and successful theory of gravity. General relativity casts gravity as the result of curvature in space and time, and a black hole is a region of space and time (or "space-time") that becomes so steeply curved that nothing falling into the region will ever be able to climb back out. A spherical boundary marks the region from which there is no escape. Within this "event horizon," all paths lead deeper into the black hole, toward the ultimate end of the road — a point known as the singularity.
No one knows exactly what happens if a particle — or an unlucky astronaut — reaches the singularity, because general relativity fails there. Because the theory has these points of failure, physicists know that it can't be quite right. Any complete theory of gravity should be able to say what happens at the center of a black hole, even if something akin to a singularity remains.
A less familiar depiction of a black hole is suggested by certain calculations in string theory. A candidate quantum theory of gravity, string theory attributes gravity and all other forces to submicroscopic vibrating strings. In this view, the black hole isn't an empty bubble, but rather a planet of tangled strings known as a fuzzball. An astronaut would go splat against a fuzzball rather than simply falling into it. Fuzzballs have some theoretical upsides, namely that they don't have unknowable singularities or sharp, inescapable horizons. Instead, they have ever-so-slightly fuzzy surfaces, which let them sidestep a thorny paradox in black hole physics posed in the 1970s by Stephen Hawking.
In "loop quantum gravity," which posits that space and time are made from a fine mesh of loops, the structure of space-time prevents gravitationally collapsing matter from reaching sufficient density to form a singularity. Instead, it forms a super-subatomic orb called a Planck star. And in "quadratic gravity," a lesser-known theory treating gravity like the other forces, matter collapses into a "2-2-hole," which an astronaut could conceivably climb out of if they had trillions of years.
While these portrayals of black holes differ dramatically from each other in concept, their physical distinctions — the ones you might hope to pick up in an experiment — are essentially impossible to detect. A recent tour-de-force analysis of vibrations in space-time emitted by colliding black holes found no sign of any deviations from the classic black hole of general relativity. But the search wasn't sensitive enough to rule out the fuzz of a fuzzball or any of the above theories' other predictions.
For that reason, many physicists don't worry about the fundamental nature of black holes at all. Many astrophysicists dedicate themselves to a different question: the role of black holes in the cosmos. In that quest, they have puzzled over the appearance of a mysteriously small black hole and an impossibly big one. They've spotted bubbles towering over our Milky Way galaxy, presumably inflated by a violent eruption from our friendly neighborhood supermassive black hole (which they have photographed). And they have tried to work out how a minuscule black hole can heat a vast galaxy.
After a string of unexpected discoveries of black holes too big for their tiny galaxies, black holes too big for their age, and just last month a black hole too big and "naked" to have been formed in the normal way (when a star collapses), it's becoming increasingly clear that the universe has all kinds of recipes for cooking up black holes, whatever they may be. | 
