Authors:

You've heard me talk about the quantum realm in Ant-Man which was certainly designed for that movie and for that story to take the notion of somebody who has the ability to shrink to another level we've never seen before, but as we were doing that, and studying it and talking to the science advisors who are always more than willing to spend an afternoon with us and talk about these amazing things - the quantum realm is another dimension.

Source: screenrant.com
You've heard me talk about the quantum realm in Ant-Man which was certainly designed for that movie and for that story to take the notion of somebody who has the ability to shrink to another level we've never seen