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.
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