Learn French: Le détective observe quelque chose. - The detective observes something.

Even though it is written as two words (literally "some thing"), quelque chose acts as a single indefinite pronoun meaning "something". You use it as a single block of meaning exactly where you would normally put a noun, such as right after the verb observe.

The word chose is feminine. Does that make quelque chose feminine too?

Surprisingly, no! Even though une chose (a thing) is feminine, the pronoun quelque chose is always treated as masculine singular. If you wanted to describe the "something", you would use a masculine adjective and connect it with de. For example: Le détective observe quelque chose d'intéressant (The detective observes something interesting).

If I wanted to say the detective observes nothing, would I just put ne... pas around the verb?

Not quite. To say "nothing", you replace the affirmative pronoun quelque chose with the negative pronoun rien. The sentence would become: Le détective n'observe rien. You drop quelque chose entirely, because "something" and "nothing" are opposites.

Could I use the verb regarde instead of observe here?