Black Holes start out small. But they attract enemies. Enemies which collide with it get absorbed and the Black Holes get bigger. If it gets big enough, a Black Hole will explode, releasing a flurry of Seekers and Butterflies which will attack you.
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If you shoot a Black Hole, it will shrink and eventually pop harmlessly. But the larger the Black Hole, the more powerfully it will repel your fire, so you need to go closer to it for your shots to connect. And also, the larger the Black Hole, the more powerfully it will ''attract you''. So sitting on the edge while shooting one is ''dangerous''.
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But here's the kicker. The points value of a Black Hole is '''150 + (5/2)N(N+1) points * your current multiplier''', where N is ''the number of enemies who have fallen into the Black Hole''. That number, you'll notice, goes up with the ''square'' of the number of enemies. If you let enemies fall in, then shoot the Hole back to a manageable size, and repeat, you can get up to serious scores. I mean:
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* 10 enemies = 425 points
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* 100 enemies = 25,400 points
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* 1000 enemies = 2,502,650 points
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"Farming" Black Holes in this way is an amazingly difficult. Sudden floods of enemies or forced Bomb usage can wipe out your plump, juicy Black Holes before you have the chance to cash them in at a moment's notice.