Ahrm, we call it dark matter.

Spiral galxies are not exploding. We can measure their kinematics directly from spectral line shifts.

Someone has proposed altering the Law of Gravity. This theory is calle MOND (modified newtonian dynamics). It fits the data well, but has no reason to be the soloution.

Why should we be able to "see" all of the matter in a galaxy?. The only things we can see are the parts that are producing light. There are many ways to package matter so that it does not emit, cold gas, black holes, lots of small planets.

How about something we know exists but is just very hard to detect? a neutrino for example. If there are lots of neutrinos in the galaxy we won't be able to detect them.

The moniker "dark matter" just refferes to objects whoose existence can only be inferred through their gravitational effects. Pluto was discoverd due to its effect on the orbit of Neptune, (i think). Until it was observed with a telescope Pluto was Dark Matter