When you watch the following video https://youtu.be/PWDPRjWPXh0?is=JmuV8pAmj1r7SgCT
you hear a crunching-sound when the rocket hits the dome. Similiar to how it would sound when a solid rock hits ice.
The sun is responsible for the existence of the dome. Its heat created the space under the dome AND the oceans (sun melted a lot of ice).
Where the suns heat is not strong enough we have ice.
Rainbows are prolly caused by prism-like surface of the dome. A prism made out of ice is also able to seperate sunlight.
Operation Fishbowl or Dominic were done to see if there really was a ice-dome in the sky. After finding it in Antarctica they probably wanted to see how far up it goes. By firing rockets at it you can see at which height your reach the ice-dome depending on your location on earth.