In cold, dense molecular clouds, what state does the gas predominantly exist as?

Molecules, such as $ ext{H}_2$ and CO.

The state of matter within a nebula is highly dependent on its local environment, particularly temperature. In cold, dark molecular clouds—the regions where star formation is currently beginning—the temperatures are extremely low, sometimes reaching only 10 Kelvin above absolute zero. Under these frigid conditions, atoms bind together readily to form stable molecules. Therefore, the gas exists primarily as molecular hydrogen ($ ext{H}_2$) and carbon monoxide (CO), which contrasts sharply with the atomic or plasma states found in hotter nebulae energized by young stars. While ice mantles form on dust grains, the gas component itself is molecular.