Space was less empty when it was younger, so I suppose you could say that the answer to your question is "because it's over 13 billion years old."
But really, "why" questions like this come with too many hidden assumptions to be helpful. You need to clarify why you expect it to not be empty. The way I see it, you could just as easily ask why is space so full of stuff as it is.
But really, "why" questions like this come with too many hidden assumptions to be helpful. You need to clarify why you expect it to not be empty. The way I see it, you could just as easily ask why is space so full of stuff as it is.