Diminutive they may be, but the smallest galaxies seem most able to muscle out visible matter, and so are darker than their larger cousins. This deepens a mystery about where all of the universe's visible matter has gone.
Ordinary matter – the zoo of protons, electrons, and other particles we see around us – is thought to make up just 4 per cent of the universe, with the rest being dark matter and dark energy. But inventories of the stars and gas in the nearby universe have revealed
Now there's another twist in the mystery: it seems pint-sized galaxies hold even less matter than expected. The smaller a galaxy, the smaller its proportion of normal matter to dark matter, says Stacy McGaugh of the University of Maryland in College Park, who presented a census of the contents of galaxies at a meeting of the American Astronomical Society in Washington DC last week.
Why so? One possibility is that stellar explosions blast normal matter into intergalactic space. Since smaller galaxies would have a weaker gravitational hold on this ejected material, they could end up dominated by dark matter, which would be unaffected by the explosions.
A more unconventional explanation is that the difference between small and large galaxies is illusory. Instead, the motions of stars in the galaxies as observed might be explained if dark matter does not exist but gravitational laws deviate from Einstein's theories on large scales.
Journal reference: Astrophysical Journal, DOI 10.1088/2041-8205/708/1/L14
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