When the structure of the atom was first being studied, people had no idea what they would find, or even that atoms existed at all. However, after many experiments, it was found that atoms are built of smaller components, some of which are charged (protons and electrons).

It was also discovered that the charge of the electron was exactly equal and opposite to the charge of the proton. Since the overall atoms were uncharged, the number of protons and electrons had to be the same in each atom. However, some atoms gain or lose electrons, and thus become charged. These charged atoms are called ions, and they don't have equal numbers of protons and electrons. For example, common table salt, sodium chloride, is made of sodium ions and chloride ions. Sodium ions have 11 protons and 10 electrons; chloride ions have 17 protons and 18 electrons. But, a fundamental law of physics--the law of charge conservation--states that the total amount of charge in a system always stays the same. So, in any uncharged system, the sum of charges must be zero. Since the universe is thought to be uncharged, that means that the total number of protons in the universe must equal the total number of electrons.