It's a common misconception that space is truly empty. It's often called a vacuum. A true vacuum is an volume of space that has NOTHING in it. Not a single particle exists inside a true vacuum. Although space is very empty, it is not completely empty. Between stars in space there is what is called the interstellar medium (ISM). The ISM is composed of gas and dust and makes up the "stuff" between stars. The density of the ISM varies depending on location in our Galaxy, but on average the density of the ISM is 1 atom/cm^3. This means that if you take a tiny cube that is 1 cm on each side, the average ISM of this volume will have a single atom in that cube. That's not much and it might be treated as a vacuum, but it isn't a true vacuum because that single atom resides in the cube.
How does this compare with air density on Earth? Much, much different! The average air density on Earth is 2.5 x 10^19 particles in a single cubic centimeter. That's a 10 million trillion particles in that tiny cubic centimeter, but without that large number we would all die due to lack of air!
I may be acting a bit picky regarding the true vacuum comment, but too many people think space is empty and that simply is not true.
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