# Why is molecular hydrogen (H2) so difficult for astronomers to detect?

I am reading a great deal about the various forms of hydrogen throughout the universe, and I keep reading that 'molecular hydrogen is notoriously difficult to detect', and other sentiments along those lines...

Why, exactly, is molecular hydrogen gas so difficult to detect?

• And there is UV absorption from H$_2$. – ProfRob Sep 12 '20 at 10:32