eli5: Why are rain clouds darker than normal clouds?

Question

Title says it. I know they sometimes aren’t but what causes them to be so dark sometimes?

In: Physics

Answer 1

Rain clouds are full of rain…. the denser they are the less light goes through and they appear darker. This is opposed to light fluffy clouds which are less dense.

https://www.livescience.com/39069-why-are-rain-clouds-dark.html