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Missing matter: Where did half the universe go?

Forget dark matter – a vast amount of normal matter visible in ancient gas clouds has gone AWOL. Now astronomers are finding clues to where it’s hiding

WHEN Isabelle Grenier surveys our galaxy, she sees things that aren’t there. Atoms, specifically. Atoms that are present when she looks into deep space, to regions seen as they were just a billion or so years after the big bang, and which should still be in our cosmic neighbourhood today. Except they aren’t. “We lose them,” says Grenier. “We see all this atomic matter in the past, but not around us now.”

Forget dark matter, dark energy or any other hypothetical substance postulated to plug gaping holes in the fabric of the universe. Here is a tangible scandal of cosmic bookkeeping right on our doorstep. When we tot up all the everyday atoms in our galaxy - the sort that make up its stars, planets and people - about half of what we expect to see is missing.

Grenier, an astrophysicist at CEA Saclay in Gif-sur-Yvette, France, intends to track it down. In the past few years, she and others have started to see some of the missing matter: hidden pockets of extremely cold matter all but invisible to conventional telescopes. Problem solved? Not a bit of it. The new entries in the cosmic ledger leave us a long way from balancing the books, and are raising questions of their own. They could cause a major recalculation of how stars live and die, how galaxies continue whirling round, and even how they come to be in the first place.

Counting the atoms in the cosmos might sound a pointlessly laborious task. It is anything but. The particles that make up atomic nuclei formed shortly after the big bang 13.7 billion years ago, and their numbers depend sensitively on what exactly went on back then. If we look at the far cosmos, seen as it was 10 to 12 billion years ago, we see the number of atoms our favoured theories of our cosmic origins predict, sitting quietly in clouds of hydrogen and helium gas. So we would expect to see the same numbers in our neck of the woods today.

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Filed under universe space dark matter astronomy galaxy stars cosmology particle physics theoretical physics

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