Intergalactic space (Earth-2042)

"That's... our galaxy. We're beyond the rim."

- Obi-Wan Kenobi to John-117, on Installation 00

Intergalactic space is the region of space between galaxies. The volume of intergalactic space is mostly empty and has an extremely low density. In rare cases, naturally occurring objects such as stars may be found outside galaxies. The astronomical objects closest to the galaxy are the three small satellite galaxies known as the Rishi Maze, the Companion Besh, and Path Kethona, the latter which was believed by the Forerunners to have been most likely the point of origin of the Flood.

The Forerunners were capable of effortlessly traveling into intergalactic space, and even constructed one installation there, known as Ark. The Ark would become the site of the final battle of the Clone Wars in December 19 BBY. For purposes other than the use of the Arks, the Forerunners rarely ventured outside the margins of the galaxy, as there were no resources of interest.

Source

 * https://www.halopedia.org/Intergalactic_space