Nice art. I would balance difficulty more. You can get a lot of time by just not killing last enemy. There is nothing that would push player further that much.
I would change:
1. Center Hole should be getting smaller quicker, maybe some "charge shot" for that? Shot like that would be pointless against enemies but would hit blackhole better.
2. Game should reward killing more, in gameplay. I had no idea I have points for killing too.
Other than that, great game