Right, a theory. But if you can't count it, can't measure it, does it really exist?
But we can measure it. Its gravity reveals its existence, its quantity, and its location. So yes it exists. We just don't know what it is, and the detector experiments are testing theories about what it may be.
We also have pretty good estimates of the density of dark matter in the solar neighborhood. It amounts to 0.49 ± 0.13 GeV cm3. This means, if you weight 70 kg, your body contains about 34 trillion electron-volts of dark matter (or 6*10^-20 grams).