What is a vacuum? Is it matter?
A vacuum, to us, is a space with no matter in it. As a practical matter though, it's really a space with very little matter in it. You might already know that it's REALLY hard to get all the matter out of any space. Believe it or not, vacuums are very important and are becoming more useful every day. There is actually a whole branch of science dedicated to creating and studying vacuums.
Many modern devices (like the integrated circuit chips that make everything from cars to computers work), have to be fabricated in a vacuum. Jefferson Lab uses vacuums for thermal insulation. A lot of our equipment will only work at extremely cold temperatures. We operate at 2 degrees above the lowest possible temperature in the universe - you bet we're paying attention to insulation! If you could insulate your home with the same insulating vacuum that we use for our accelerator then you wouldn't need a furnace at all!
Even outer space, which is considered a vacuum and has less matter in it than anything mankind can reproduce, still has some atoms bouncing around.
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)