About 2500 years ago, Democritus wrote his philosophy that the universe was made up of atoms without any proof of their existence. For the past couple hundred years, chemists has been using the atomic theory without actual proof atoms, because it worked in practice. Today, we have electron microscopes as shown in this picture that can see atoms. It is no longer a philosophy, but a theory. Atoms are so small that light microscope still cannot see the atoms, it take beam of electrons to see atoms. It is interesting that it takes a microscope using beams of small pieces of an atom, electrons, in order to see the atoms itself. Electrons are more than a 1000 times smaller than an atom.