I’ve noticed them quite a few times myself and wondered what they were. Good to finally have an answer.
The first few minutes are enough to find out, but to briefly explain what they are for those not interested in watching the video, antibubbles are just like regular bubbles but inverted. Regular bubbles consists of a thin layer of liquid encasing a gas. An antibubble is a thin layer of gas encasing a liquid, typically one different from the liquid it is surrounded by.