Half of the universe's hydrogen gas, long unaccounted for, has been found

Astronomers tallying up all the normal matter--stars, galaxies and gas--in the universe today have come up embarrassingly short of the total matter produced in the Big Bang 13.6 billion years ago. In fact, more than half of normal matter--half of the 15% of the universe's matter that is not dark matter--cannot be accounted for in the glowing stars and gas we see.