The Local Bubble is a cavity in the local interstellar medium (ISM) at least 300 light years across containing a neutral hydrogen density that is approximately one tenth of that of the average ISM in the Milky Way (approximately 0.05 atoms per cubic centimetre, as opposed to 0.5 atoms per cubic centimetre). The hot diffuse gas in the Local Bubble emits X-rays.

Most astronomers believe that the Local Bubble was formed between some hundreds of thousands to a few million years ago by nearby supernovae that pushed aside gas and dust in the local ISM leaving behind hot, low density material.

The Local Bubble is not spherical, but seems to be narrower in the galactic plane, becoming somewhat egg-shaped or elliptical, and may widen above and below the galactic plane, becoming shaped like an hour glass.

The Local Bubble abuts other bubbles of less dense ISM, including, in particular, the Loop I Bubble. The Loop I Bubble was created by supernovae and stellar winds in the Scorpius-Centaurus Association , some 500 light years from the Sun. The Sun, with a few other stars, is embedded in a denser cloud of ISM known as the Local Fluff which formed where the Local Bubble and the Loop I Bubble met. The gas in the Local Fluff has a density of approximately 0.1 atoms per cubic centimeter.