Interaction between a dwarf galaxy orbiting around a larger galaxy with 100 times its mass. The upper left panel shows the two dwarfs approaching one another. The middle panel gives the state of the system after 2 billion years, and the right panel shows the appearance of the galaxies after 7 billion years. Bottom Row: Shown is the orbit of the same small galaxy (in white) around the Milky Way today (in yellow), which has 10,000 times its mass. Credit: E.D'Onghia, Harvard Center for Astrophysics

Close encounters of the galactic kind may explain the existence of an unusual type of dwarf galaxy, a new study suggests.

So-called dwarf spheroidal galaxies are small and very faint, containing few stars relative to their total mass.

These star-deprived galaxies appear to be made mostly of dark matter — an elusive form of matter detectable only by its gravitational influence. Dark matter outweighs normal matter by a factor of five to one in the universe as a whole.

Astronomers have found it difficult to explain the origin of dwarf spheroidal galaxies.

"These systems are 'elves' of the early universe, and understanding how they formed is a principal goal of modern cosmology," said study author Elena D'Onghia of the Harvard-Smithsonian Center for Astrophysics (CfA).

Previous theories have required that dwarf spheroidals orbit near large galaxies like the Milky Way — but this doesn't explain how the dwarfs that have been observed in the outskirts of the "Local Group" of galaxies (which includes the Milky Way) could have formed.