Another evolutionary link discovered

If it has mass, it will have a gravitational force. This can be used to indirectly observe an object, sometimes rather accurately. This is one of the ways we find new planets... we watch for a star to "wobble", showing an outside gravitational force acting upon the star. This can be used to show mass, distance, and orbit time, among other things. All without directly seeing the planet. That's not to say we haven't "seen" a planet, but it is a good definition of indirect observation.

If it has gravitational force, doesn't there need to be a center to gravitate too? I would say effect, not force.

???

Gravitation is brought on by mass, and that's all that is needed to exert a gravitational force. The more mass, the more gravitation.

Every object with mass has a center.

I swear that's Drnaline, he had a thread about Dark Matter once.