By calculating the gravitational force generated by that point. Since we can see things around it being drawn to it we can calculate those masses by terms of other gravity forces, spectrography, and some other calculations...calculate the speed and centripetal force and voila! Mathematically we can show black holes exist as well, through some serious physics.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth