Constellation-X will search for the missing baryons trapped in the channels of dark matter that connect galaxy clusters, the largest known structures in the Universe. In the early Universe, we see much more hydrogen than we do today because the hydrogen was cold. Clouds of cold hydrogen absorb light that passes through it. The more hydrogen, the less light that passes through. The gas also shock-heated as it collapsed with the force of gravity.
Hot hydrogen is harder to see.
Light passes through it without being absorbed as much as when it was colder. These elements, which constitute perhaps only 1 percent of the missing baryons, tells us how much hydrogen is out there. These absorption lines are very faint, though. Hubble has seen traces of one type of oxygen isotope. Constellation-X will be sensitive enough to detect faint absorption lines from several isotopes of several different elements, providing tighter constraints in calculating the total amount of missing hydrogen.