Well, black holes acquired their spin the same way you did!
If you're thinking, "What! I'm not spinning. I'm firmly seated (or lying)," well, think again.
Although you may be static, the planet you're on is spinning. And rapidly, too. It's spinning around itself.
And it's taking you for the ride.
It gets more interesting the more you think about it. Our Sun is spinning too. So is the moon, other planets, comets, asteroids, and pretty much everything else in space.
But why?
We Live in a Merry Go 'Round Universe
Image Credits: https://www.jpl.nasa.gov/news/nasas-nustar-helps-solve-riddle-of-black-hole-spin/
It's not hard to see why, you know.
Think of any object in space – an asteroid, planet, star, or whatever you can imagine.
In this case, let's begin with a star.
Let's say, for our argument's sake, that it's currently still and not moving in any way. It doesn't even have a spin of its own.
How long do you figure it can keep up its act?
Sooner or later, something will hit it. It could be something big, like another star, or something small like a comet.
In fact, everything in space is constantly bombarded with stuff. It could be small stuff like comets and rocks. Or it could be big stuff, like a planet-sized body or another star.
Chances are those objects won't hit our star head-on or perpendicular to its surface. The overwhelming majority of them will hit our star at an angle.
They might even trace a hyperbolic path as they collide into our object.
It's like someone accidentally hitting you in the right shoulder as they pass you by. Even without your volition, your body turns in the direction of "their path" due to the sheer force of the impact. In other words, you "spin."
If you were in space, there would be nothing you could do to stop this spin. Fortunately, gravity binds you to Earth, and the friction between your feet and the ground helps you transfer that spin (angular momentum) to Earth.
Our star isn't so fortunate. It gains the spin and keeps it, until something else hits its left shoulder…err…I mean, in the other direction, either slowing it down or making it spin the other way.
Every small thing – no matter how infinitesimally small it is – hitting a star ends up affecting its spin, even if it's by an immeasurable amount, because its energy must be conserved.
This applies to every object in space.
So, nothing in space is ever without spin.
Their spin could be very, very small. But it's always there. Although theoretically it's possible to have an object in space with no spin, it's practically (probabilistically) impossible to have one.
In fact, most stars develop their spin when they're born because the collapsing matter that created them fell together in complex, turbulent, and spiral paths.
So, when these stars eventually die and create black holes, the latter merely retain the spin they inherited from their parent stars.
Now, since they're much smaller in size due to extreme compression, their spin is much, much faster than that of their stars.
I'm sure you've come across the analogy of figure skaters spinning faster when they draw their hands in. So, I won't go into it any further.
To put it simply, black holes spin because everything in the universe is always spinning, too!
So, the next time you're overwhelmed by life and go, "My head is spinning," don't worry. It's the universe and everything in it that's gone bonkers and spinning! Not just you.