How to create the rounded object behind the circles in Adobe Illustrator?
SURELY there is a better way to do this, and I'm going to follow this to see what people suggest. But, this is 9/10, what I would do:
Basically, I would create a shape with the pen tool as I've done in step 2 and then mirror it like I've done in step 3. There's nothing too mathematically pleasing about this solution (I feel like there's probably a sounder solution with the pathfinder tool) but it works.