My sand is turning dark

Please help, I’ve had my betta now for two weeks and I just noticed that one side of my sand is turning dark. I read a forum saying something like this can be caused by Anaerobic pockets in the sand. What should I do? Do I remove this chunk of sand completely? Do I remove all the substrate and switch it to gravel instead of the sand? This is my first time having sand in a fish tank and I really would appreciate any suggestions. Thank you for your time.

If you never clean your substrate, it will turn darker over time. Water and debris move through the sand continuously, albeit slowly. If you have wood, botanicals and plants, the darkening goes a lot quicker. Bubbles might escape from the sand substrate if you would disturb it, which can be CO2 or also O2 if you have plants rooted.

Your sterile display becomes "a slice of nature". It is beautiful.

Regarding Anaerobic pockets, I compare it with personal issues. If my arm aches and I try to diagnose the cause online and look for images, I better go and visit the hospital as quickly as possible.... .

I think the darkening is simply a mixture of algae & bio gunk. Rake the sand periodically. That should eliminate the problem & ease your concern over subterranean gas pockets.