It has two primary constituents - the solar wind and planetary magnetospheres. It is an electrified gas, or plasma, which consists of free electrons and free atomic nuclei. A candle flame is an example of plasma.
Planetary magnetospheres are the other important constituents of space.
A magnetosphere is the region of space surrounding a planet which is dominated its magnetic field rather than the solar wind. When the solar wind encounters a planetary magnetic field, the magnetic field deflects the solar wind, and forms a protective cocoon around the planet called the magnetosphere. The solar wind shapes the magnetosphere, compressing it on the sunward side and drawing it out to a long tail on the nightward side. Earth's magnetosphere is especially important because it protects the planet from the potentially lethal particles of the solar wind which could be damaging to many forms of life. On Earth, the outer core is composed of a highly conductive liquid alloy of iron and nickel.