No, it's not quite "no guess at all". Using gravitational lensing to estimate how much mass there is in a part of a galaxy is pretty smart. It turns out that

No, it's not quite "no guess at all". Using gravitational lensing to estimate how much mass there is in a part of a galaxy is pretty smart. It turns out that there sometimes indeed is mass (because the lensing is there) even though it doesn't seem to give off much light. (But I don't really believe in dark matter, either. It's mostly epicycles and phlogiston.)