Dark Energy, if not an anti gravity, what is it? There are 4 main theories and my take makes 5.

My take is that dark E is a balancing space energy force that expands as matter and dark matter continues to concentrate. It will stretch only so far until an ENTIRE PHASE CHANGE occurs reducing the universe to a lower energy state as entropy predicts.

As for how we see a diffraction pattern, we thought as we put constraints on direction and positions simultaneously it collapses wave possibilites. We are really seeing how a spectrum of time can show something different. -or not- I'm not at my peak right now.
I see what you are saying but air molecules have almost nothing to do with it.

If anyone here does not know about the 'dual slit' experiment
Prepare to be mystified - GOOGLE IT