Here's a quick grab I made, off the Internet, to give you some idea of what I mean, when I say that matter is "mostly" empty space. Below the writer uses the hydrogen atom to illustrate. Hydrogen is used as a proxy for "all matter", because it is the most abundant element in the universe.
The most abundant element in the universe is hydrogen. The hydrogen atom, like all atoms, is composed of a nucleus orbited by a(n) electron(s). The most likely orbit of the electron in a hydrogen atom, according to the Bohr model, is about 5.3 X 10-11 meters. The volume of the hydrogen atom then is about 6.24 X 10-31 cubic meters. The single proton that makes up the nucleus of the hydrogen atom has a volume of about 4.19 X 10-42 cubic meters. Hydrogen's single electron's volume is approximately 9.2 X 10-44 cubic meters. If we compare the sum of the volumes of the electron and the proton to the volume of the entire hydrogen atom, the proportion of that atom that is empty space is 99.999999999314%!
So that's your back-of-the-envelope figure. Matter is generally 99.999999999314%