Most of the universe is empty space. Even you, no matter how big or small, are mostly empty space. But the universe get a bit self-conscious about all its unused space and creates what physicists call quantum foam. The empty space is, at the Planck scale, actually made up of particles that appear and disappear in a span of time that’s long enough to be measurable — though this is quantum physics, so it’s nanoseconds — but short enough to have absolutely no bearing on the existence of the universe.
It’s called quantum foam because all of this exists in a similar structure to the way carbonation works in a pint of beer. A bubble exists for a time and then disappears and another one forms and vanishes somewhere else. It doesn’t affect the beer in any way,* but it can be observed. The particles in empty space exist and then don’t just as the carbonation bubbles exist and then don’t. Except they do it on a very, very, very small scale. In a Planck scale of space and time (10^-35 metres and 10^-44 seconds).
And you — well, physicists — can actually see how quantum foam works in experiments using metal plates separated by a distance. Because the amount of foam between them is less than the amount surrounding them, the plates will eventually close the gap and come together.
*Well, technically speaking, it does in exactly the same way that quantum foam affects the universe, but we’re not going into higher concepts of beer and physics here today.