Empty space is nothing but. According to the weird rules of quantum mechanics, it’s actually filled with an endless amount of energy, known appropriately enough as vacuum energy.
Quantum mechanics is super weird. One of the lessons of the quantum is that particles, like electrons and photons and neutrinos and whatnot, aren’t what they seem to be. Instead, each of the particles that we see in nature is actually just a piece of a much larger, grander entity. These grander entitles are known as quantum fields, and the fields soak every bit of space and time, all throughout the universe.
There is a quantum field for every kind of particle: one field for the electrons, one for the photons, and so on. These fields are invisible to us, but they make up the fundamental building blocks of existence. They are constantly vibrating and buzzing. When the fields vibrate with enough energy, particles appear. When the fields die down, the particles disappear. (Another way to look at this is to say what we call a “particle” is really a localized vibration of a quantum field). When two particles interact, it’s really just two pieces of quantum fields interacting with each other.
So if you think about all the particles that make up your body, all the electrons and protons and neutrons, those particles are just local vibrations of entities that span the entire cosmos. Neat.
These quantum fields are always vibrating, even when those vibrations aren’t strong enough to produce a particle. If you take a box and empty out all of the stuff – all the electrons, all the photons, all the neutrinos, all the everything – the box is still filled with these quantum fields. Since those fields vibrate even in isolation, that means that the box is filled with invisible vacuum energy, also know as zero-point energy – the energy of these fundamental vibrations.
This means that the vacuum of the universe is really made of something. There’s no such thing as a true vacuum; wherever you go, there are always vibrating quantum fields.
We know that the vacuum energy – this incessant boiling and vibrating of the quantum fields – interacts with the everyday world. For example, electrons inside of atoms occasionally move to more distant orbits away from the nucleus. There’s no reason for the electrons to make their way back down, until they get jostled by the quantum fields, prompting them to fall back into their normal orbital place.
I know this all sounds weird and new-agey, but it’s a real thing. Everywhere you go in the universe, there’s always something.