By definition, 'empty' means devoid of matter, or lacking in substance. So when most people hear the term empty space, they assume that, indeed, space is entirely empty. Once upon a time, we thought this was true, but these days, we know that space isn't a perfect vacuum. It, in fact, contains things we can't even conceive of, like virtual particles, which pop in and out of existence at a black hole's whim.
Learn more about spacetime itself and the particles inhabiting it in the video below: