WHAT IS NGC 1015-DF2 AND DOES IT CONTAIN DARK MATTER?
NGC1052-DF2, or 'DF2' for short, is 65 million light years away and one of a newly recognised family of 'ultra-diffuse' galaxies.
They are thought of as 'ghostly' because they contain so few stars, making them very faint despite their large size.
DF2 is about the same size as the Milky Way but contains 200 times fewer stars, as well as almost no dark matter.
The weird object was first spotted by astronomers using the Dragonfly Telephoto Array, a special 48-lens telescope in New Mexico, US, designed to find ultra-diffuse galaxies.
Follow-up observations revealed a number of unusual characteristics. Unlike typical spiral galaxies, DF2 had no dense central region, and lacked spiral arms or a disc. And unlike elliptical galaxies it had no central black hole.
But the oddest thing about DF2 was revealed when astronomers estimated the galaxy's mass by measuring the speed at which clusters of stars were orbiting its centre. Slower velocity equated to greater mass.
They found that almost all of DF2's mass could be accounted for by visible stars, gas and dust. The expected 'invisible' mass consisting of dark matter was not there.
'If there is any dark matter at all, it's very little,' said Prof van Dokkum. 'The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter.'
Describing DF2 as 'astonishing', he said the galaxy was so sparse it was possible to see right through it.