A neutron is a sub-atomic particle with no net electrostatic charge, with a very similar mass to a proton. Neutrons are thought to comprise of one up quark of charge +2/3 and two down quarks of charge -1/3 each, resulting in a net charge of zero. Neutrons are present in almost all atomic nuclei except for Hydrogen.
When neutrons are isolated in space they decay with a half life of about 15 minutes and decay into a proton, an electron and an anti-neutrino. This process is known as beta decay, where beta refers to the electron. As a result of this process, space is relatively devoid of free neutrons.
The neutron was discovered by Chadwick in the 1930s.