Widely accepted theories of dark matter,  a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts, expect the solar neighborhood to be filled with the stuff - but it isn't, at least as far as can be detected.

This extra ingredient in the cosmos was originally suggested to explain why the outer parts of galaxies, including our own Milky Way, rotated so quickly, but dark matter now also forms an essential component of theories of how galaxies formed and evolved. Today it is widely accepted that dark matter constitutes about 83% of the mass in the Universe, though it has resisted efforts to clarify its obscure nature. All attempts so far to detect dark matter in laboratories, on   Earth, have failed.

A team using the MPG/ESO 2.2-meter telescope at ESO’s La Silla Observatory, along with other telescopes, has mapped the motions of more than 400 stars up to 13 000 light-years from the Sun. From this new data they have calculated the mass of material in the vicinity of the Sun, in a volume four times larger than ever considered before.

Milky Way galaxy dark matter missing
Milky Way galaxy. Conceptually, the blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter. New measurements based on the movements of stars show that the amount of dark matter in this region around the Sun is far less than predicted and have indicated that there is no significant dark matter at all in our neighborhood. The blue sphere centered on the Sun’s position shows the approximate size of the newly surveyed volume, but not its precise shape. Credit: ESO/L. Calçada

“The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun,” says team leader Christian Moni Bidin of Departamento de Astronomía, Universidad de Concepción, Chile. “But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!”

This may mean that attempts to directly detect dark matter particles from terra firma are unlikely to be successful.

By carefully measuring the motions of many stars, particularly those away from the plane of the Milky Way, the team could work backwards to deduce how much matter is present. The motions are a result of the mutual gravitational attraction of all the material, whether normal matter such as stars, or dark matter. Astronomers’ existing models of how galaxies form and rotate suggest that the Milky Way is surrounded by a halo of dark matter. They are not able to precisely predict what shape this halo takes, but they do expect to find significant amounts in the region around the Sun. But only very unlikely shapes for the dark matter halo — such as a highly elongated form — can explain the lack of dark matter uncovered in the new study. 

Theories predict that the average amount of dark matter in the Sun’s part of the galaxy should be in the range 0.4-1.0 kilograms of dark matter in a volume the size of the Earth. The new measurements find 0.00±0.07 kilograms of dark matter in a volume the size of the Earth. The new results also mean that attempts to detect dark matter on Earth by trying to spot the rare interactions between dark matter particles and “normal” matter are unlikely to be successful.

“Despite the new results, the Milky Way certainly rotates much faster than the visible matter alone can account for. So, if dark matter is not present where we expected it, a new solution for the missing mass problem must be found. Our results contradict the currently accepted models. The mystery of dark matter has just become even more mysterious. Future surveys, such as the ESA Gaia mission, will be crucial to move beyond this point.” concludes Christian Moni Bidin.

Citation: C. Moni Bidin, G. Carraro, R. A. Méndez and R. Smith, '“Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood', The Astrophysical Journal (upcoming) (preprint)