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A neutron is a type of baryon that has no electric charge. (A baryon is a hadron, i.e., a composite particle, that is made up of three quarks.) Neutrons are subject to the strong force, the force which draws together protons and neutrons to form an atomic nucleus as well as holds the three quarks of a neutron or proton together. Other than 1H, all nuclei include neutrons. Neutrons and protons have roughly the same mass (around 940 MeV), which is about 1800 times that of an electron (around 0.5 MeV).
Neutrons can be stable when bound to protons, but free neutrons (not so-bound) decay (free neutron decay) with a half-life of roughly 610 seconds, i.e., mean lifetime of roughly 880 seconds (the free neutron mean lifetime, which is often shortened to neutron lifetime in context). This is one reason we are not bombarded by free neutrons and neutron capture does not occur from neutrons produced at astronomical distances. Measuring the neutron lifetime has been a challenge and determinations are of current research interest because two types of experiments produce decidedly different results (though both are in the general vicinity of 880). The exact value has cosmological implications.
Neutrons were created very soon after the Big Bang when the temperature dropped sufficiently that quarks combined into protons and neutrons. Radioactive decay sometimes converts a neutron within a nucleus to a proton or vice versa.