Astronomers have one more reason to scratch their heads over the unseen material known as dark matter. Observations of two dwarf galaxies, Fornax and Sculptor, show the dark matter within them is spread out smoothly rather than heaped into a central bulge, contradicting cosmological models.

Researchers know dark matter comprises a far greater percentage of the universe than the ordinary matter making up things like people and stars. Because of this, the distribution of dark matter determines the structure of the cosmos. Galaxies form when they are attracted to and anchored by large clumps of dark matter.

The dwarf galaxies Fornax and Sculptor are themselves made of 99 percent dark matter and only 1 percent normal matter. It is impossible to directly see the dark matter but, by observing the rotation of stars around each galactic center, researchers can detect its influence and map out its distribution.

While simulations suggest that the dark-matter density should increase sharply near the galactic centers, the recent observations found the dark matter spread relatively uniform throughout. Yet if these dwarf galaxies have no “clump” in their center, then what is pinning them in place?

Observations of other small galaxies have similarly failed to find a dense central dark matter core, a difficulty that has prompted astronomers to begin expanding their ideas on the mysterious substance.

It is possible that dark matter might interact more with ordinary matter than currently thought, allowing the regular matter to stir up the dark matter and spread it out. Alternatively, dark matter might move faster than expected and therefore be less prone to clumping in galactic centers. Either case creates many further mysteries and problems for astronomers to keep mulling over.

Image: ESO/Digitized Sky Survey 2