Here's a breakdown of their characteristics:

* Dense: Molecular clouds have much higher densities than the typical interstellar medium. This allows molecules to form and persist, as collisions are more frequent.

* Cold: Temperatures in molecular clouds are very low, typically around 10 Kelvin (-441 degrees Fahrenheit). The low temperatures favor the formation of molecules.

* Dark: The dense composition of molecular clouds blocks visible light from passing through, making them appear dark in optical telescopes.

* Molecular: The most abundant molecule in these clouds is molecular hydrogen (H2), which forms due to the presence of dust grains. Other molecules, like carbon monoxide (CO), water (H2O), and ammonia (NH3) are also found.

Significance:

Molecular clouds are incredibly important in the universe because they are the birthplaces of stars. The dense gas and dust within them collapse under their own gravity, leading to the formation of stars and planetary systems.

In summary: Molecular clouds are the key ingredients for star formation and contain the building blocks for planets, moons, and even life itself.