Magnetic fields in space. Much of the visible matter in the universe is in the form of plasmas. This means that there are many electrically charged particles in motion, and therefore magnetic fields. Cosmic magnetic fields are very significant in many astrophysical contexts, such as stars, relativistic jets, pulsars, accretion disks, and cosmic radiation.

Magnetic fields in the early universe, i.e. primordial magnetic fields, are of particular interest. Constraints on such magnetic fields are provided by considering their impacts on the microwave background radiation and nucleosynthesis in the early universe. Many suggestions have been put forward to explain primordial magnetic fields but a consensus on this topic has not yet been reached.