You can never go wrong with Imagine Dragons! It's indie rock/alternative rock but always been a favourite of mine. Same with M83. Indie/rock is a great combination, and since their popular bands, it's also got pop genre intertwined in.
You should definately check out the new album by Imagine Dragons called Smoke + Mirrors. I'm currently addicted to it. 🙂
I honestly don't know of many true "Indie" bands but I'll give you some bands that I know are not well known and that I think are great (some of these don't make music anymore.) I hope you find enjoyment from these. Happy listening.