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.
What I understood is kind of the opposite–they already knew there were hidrogene clouds around galaxies but analyzed some almost imperceptibly blurry images and found they were bigger than currently thought. They’re blurry because they were taken in some wavelength not observable until now that is scattered by the ionized gas.