The graphics and music are great. You did a really good job with the atmosphere I think.
The problem is that the mechanics of the game are too simplistic and repetitive.
It doesn't seem like there is a strategy involved and it's just "punch bad guy/buy upgrades".

Too simplistic.
It would be nice to have a score system.
It also seems like there is no loosing mechanic, I stayed on the other ball for a while and nothing really happen.
I think making a match system where, for example, the first player that gets 3 points win.
That would give the player more feedback and motivation and would improve the game and make it more interesting to play.

The gameplay is simple but it works.
The graphics are pretty good, I love the music.

The controls are ok but I think the game would feel a lot better if they were a little bit tighter and not so floaty.

Reminds me a lot of descent.
I like the simple graphics and the atmosphere in general.
You really get a claustrophobic/lost feeling. The music is great too.

I enjoyed the gameplay, my only problem is that the controls are a little bit too loose.
I understand that it's a space game and you are trying to give that feeling of floating but I think the game would gain if it had slightly tighter controls.

Thank you for your feedback.
I am working on a tutorial that will be in the next version.