What happens when Antimatter interacts with a photon? So... I was just pondering the energy and particles in the universe.
It makes sense that matter is attracted to it's self by a gravitational force - it clumps and forms some kind of gravity superposition.
So what about antimatter - since charges are opposite, perhaps it also clumps together to form anti-gravity superpositions.
Is there such a thing as an anti-photon?
When a 'photon' interacts with an anti-atom, what happens?
Have there been any experiments to demonstrate any of these?
 A: 
So what about antimatter - since charges are opposite, perhaps it also clumps together to form anti-gravity superpositions.

As Red Act says in a comment, gravity is too weak to be important on the scale of individual particles. However charge does group antiparticles together. For example an anti-proton and a positron will form an antihydrogen atom. In principle any antiatom can be made, but in the lab it's hard to stop antiparticles annihilating before they get the chance to combine to form antiatoms.
Even a particle and antiparticle can temporarily form a bound state. For example an electron and positron can bond to form positronium. However this is unstable and lasts ony about 100 picoseconds before the electron and positron annihilate. Similarly a quark and antiquark can bond to form a meson, and again these are unstable and decay via annihilation.

Is there such a thing as an anti-photon?
When a 'photon' interacts with an anti-atom, what happens?

The photon is its own antiparticle. See the answers to Do anti-photons exist? for more.
The photon will interact with any electrically charged particle and it doesn't matter whether it's an antiparticle or not. So for example a photon will Compton scatter off a positron in the same way that it Compton scatters from an electron.
A: Clumps of just anti-matter will have the same gravity field around them as clumps of matter.
There was an experiment at either Fermilab or SLAC in the 1970s or early 1980s where the falling of a beam of anti-protons was measured.  I was trying to look up details on this a couple years ago, and didn't find it. But I know I read about it long ago.  Bottom line: anti-protons fell just the same as protons.  (It might have been neutrons and anti-neutrons, not sure.)    The curvature of spacetime guides antimatter and matter alike.  Just as an expert shooter must aim the gun a wee bit high to account for gravity pulling the fired bullet down, so too must physicists account for the falling of electron beams, positron beams, proton or antiproton beams, pion beams, etc. Except in practice, it's ignored due to the short flight time and small compared to imperfections of strong magnetic fields and whatnot. 
As for the field of gravitational acceleration surrounding anti-protons or other anti particles - this cannot be measured, even for regular matter.  Only for "big" things like planets, asteroids, or even balls of lead in laboratories, can the field due to the matter be measured.  We've never made or found anything bigger than an atom made of anti-matter.  
The photon is neither matter nor anti-matter.  It is its own opposite. When interacting with matter or antimatter, all it cares about is + or - charge.  If there's anything like an anti-photon, it's when a coherent beam is shifted by 180 degrees - waves canceling as in an interferometer.
