The “dead” moniker describes the fact that these galaxies no longer have the necessary cold hydrogen gas to make stars. Without the gas, there’s no clumping of matter that eventually leads to the formation of a star.
Where do cold hydrogen form in there?
Hydrogen is easily the most abundant element in the universe. It is found in the sun and most of the stars, and the planet Jupiter is composed mostly of hydrogen. On Earth, hydrogen is found in the greatest quantities as water.