Nice thought, but as the saying goes, space is really big. Stars have a tiny angular size when viewed from even their nearest neighbours (except perhaps in clusters & galaxy cores), so not much starlight actually gets intercepted by other stars, relative to the amount of light that gets absorbed by interstellar (or intergalactic) gas and dust.
And even that absorbed light isn't actually lost: it increases the kinetic energy and the temperature of the gas & dust. So some of the energy is re-radiated at a lower frequency, typically as infrared or microwave radiation. By looking at the spectrum of that radiation we can determine its temperature, and use Doppler techniques to estimate the kinetic energy of the gas & dust.
When a star intercepts light from another star, the light energy isn't destroyed. Some of the light is actually reflected, the rest of the energy is assimilated by the receiving star's atmosphere, where it will contribute to the receiving star's own light emissions.
Astrophysicists take these things into account when evaluating the brightness of galaxies. Plenty of people have tried all sorts of alternative hypotheses to dark matter, but so far dark matter is the best explanation that we have.