Why are rain clouds dark?

Comparatively Bangalore residents would have felt this often! For only reason, almost everyone doesn’t want to waste any space within their site; even for passage in between houses & to the compound gates.
For example - we might have seen few very flexibly built compound gates that can safeguard the vehicles parked on the road; which actually supposed to do when parked inside.

Better late than never! BBMP finally realized the smart people around the city, and come up with smarter plan late in 2011 that, # of car parking space must be equal to # of kitchens in the building, must be shown up in architecture approval proposal.

The reason why rainy clouds appear dark are due to their particulate density. As the clouds form from minuscule water droplets and ice crystals, before rain starts water vapor would have the higher density, the denser they are packed, it makes the cloud turn up to thicker & BIG. And hence it blocks out most of the sun light passes through; lending a darker appearance to their lower sections.

And the best supporting fact I found - visualization by flight journey. With clouds on an overcast day seemed grayed out when viewed on the runway, yet a bright white above the cloud.
Essentially, the deeper & denser a cloud formation is, the less light will pass through it.