Black holes may wander around the edges of the Milky Way.

A huge number of supermassive black holes can roam the universe, according to new simulations.

In fact, giant wandering black holes can represent an enormous 10% of the black hole mass of the neighboring universe "'budget", according to research. This means that galaxies such as ours could have an average of 12 invisible monsters lurking around their periphery, swallowing up everything that stands in their way.

According to the study researchers, because the number of black holes increases the more mass there is in the outer "halo" of material that surrounds galaxies, clusters of galaxies, which have heavy halos, could have even more of the ravenous wanderers.

"We anticipate thousands of wandering black holes in the halos of galaxy clusters," the researchers wrote in the study.

Just as a basket with father names can be woven around the structure carrying a stone, astronomers believe that most galaxies form around supermassive black holes. The gigantic gravitational beasts, often many millions or even billion times more massive than the sun, act as anchors for long trains of gas, dust, stars, and planets that swirl in orbit around them. Closer to the black holes, this material spirals faster and heats up, forming an accretion disk that both feeds the black hole and produces the telltale radiation that makes it visible.

Typically, the mass of these black holes cements them in the centers of their galaxies, which slowly revolve around each other in clusters called galactic groups. But sometimes, a huge force - such as a collision between two galaxies - can release a central supermassive black hole, forcing it to wander through the universe like a cosmic drifter.

Stray monsters may also be released when the fusion of two black holes is disturbed, sending one or both into flight.

To estimate how often this occurs, the astronomers ran a set of simulations called Romulus that account for all known rules about how black holes behave to trace how their orbits might evolve over billions of years.

The simulations predicted that the frequent galactic collisions of the early universe, between the time of the Big Bang about 13.7 billion years ago and roughly 2 billion years later, produced enough wanderers to outnumber, and even outshine, their galactically fixed supermassive black hole cousins.

Later, as the universe grew older, many of the loose black holes merged and were recaptured by other supermassive black holes after forming binary systems with them in the centers of galaxies, the simulations found. However, many have also remained free.

"Romulus predicts that many supermassive black hole binaries form after several billions of years of orbital evolution, while some SMBHs [supermassive black holes] will never make it to the center," the researchers wrote. "As a result, the Milky Way mass galaxies at Romulus host an average of 12 supermassive black holes, which usually wander the halo away from the galactic center."

The researcher's "next steps will be to figure out possible hallmarks of the lost invisible giants'" presence in the universe so that one day soon, we can observe them first hand.

The researchers published their conclusions in the June edition of the Monthly Notices of the Royal Astronomical Society.