A new study suggests the mysterious substance called dark matter could be made up of black holes.
Dark matter is a theoretical substance that makes up 25% of the total mass of the universe, preventing galaxies from flying apart as they spin.
But despite decades of research scientists still don't know exactly what it is.
Writing in the pre-publication blog arXiv.org, Professor Paul Frampton of the University of North Carolina claims the missing mass might be primitive intermediate-sized black holes - too small to see, but too big to evaporate due to Hawking radiation.
Frampton says none of the particles in the standard model of particle physics have the properties needed to be dark matter, that is electrically neutral, long-lived and slow moving.
"Despite a decade spent searching, nobody's produced a sample of dark matter," he writes.
So Frampton has taken a different approach.
He first determined what the entropy, or measure of disorder, of the universe should be.
Using that as an upper limit, he then estimated the lower limit by adding together the entropy in all the known black holes in the universe.
He assumed that there's a supermassive black hole at the centre of every galaxy.
"The difference must be coming from something else," says Frampton.
"There's not enough visible matter to account for the missing entropy so what's left must be the entropy of the missing dark matter.
"Primordial intermediate size black holes could be responsible for this."
But how could so many black holes have formed so early in the history of the universe?
Frampton argues, something not included in current inflation theory, which describes how the early universe grew, must have caused matter to clump together at this scale to form the extra black holes.
He says there must have been two periods of inflation.
"The first led to the large scale structure of the universe we see, and the second to the clumping that created large numbers of intermediate sized black holes."
But Professor Warwick Couch of Swinburne University in Melbourne says most cosmologists believe there has only ever been a single period of inflation.
Couch also questions Frampton's claim that none of the particles in the standard model have the properties needed to be dark matter.
"That's controversial and the fact that millions are being spent on dark matter experiments looking for WIMPs (Weakly Interactive Massive Particles) shows there could be dark particles we don't know about," he says.
Still Couch says black holes do introduce disorder into the universe, and entropy is a measure of disorder in systems.
"It also forces us to question what dark matter might be. We think of it as unseen particles, but what if those particles are just black holes?
"And if Frampton's figures are correct, they do show a discrepancy between the number of black holes we know about, and the number there should be based on entropy.
Crouch says cosmologists don't usually think of the universe in terms of entropy, but it is a good way of dealing with large numbers on a cosmic scale.