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The gas clouds are formed primarily of hydrogen blown out of galaxies and star systems by supernova explosions somewhat like our sun's solar flares and coronal mass ejections but much bigger. More gas is released and flung out during the formation of new stars in a giant version of solar wind. Less violent reactions also release hydrogen gas. Huge hydrogen bubbles form and then burst, releasing a cloud of gas.
Size
These galactic gas clouds are also immensely larger than terrestrial clouds formed from condensed water vapor. Clouds viewed through the Commonwealth Scientific and Industrial Research Organisation's Parkes telescope in Australia had masses 700 times that of our sun and were 200 light years, or several trillion miles, in diameter. Water vapor clouds on Earth by comparison are usually only a few miles across and have proportionally less mass.
Significance
Astronomers have been fascinated by gas clouds found near galaxies since their makeup was determined. As of 2011, scientists believe the hydrogen is both a byproduct and a component of the birth of stars, which appears to take place near the densely packed center of galaxies. Stars go supernova at the center of a galaxy and blast huge amounts of gas outward toward the arms. The gas coalesces into clouds that then fall back toward the center of the galaxy to begin the cycle again.
Prevalence
A total number of gas clouds is unlikely to be arrived at. However, the CSIROS observers in Australia studied nearly 650 hydrogen clouds and bubbles in the Milky Way. It's likely the process is repeated in more distant galaxies many light years distant. Galaxies are the birthplace of stars that are added to the universe on a continuing basis.