There appears to be an upper limit to their numbers.
Large numbers of microstars/megasolitariy planets would create gravitational lensing events when they passed through the line of sight between us and the stars they occulted. These events have indeed been observed, but not in large enough numbers to suggest these objects dominate the stellar population.
Still, these observations are difficult to carry out, involve many risky assumptions, and have large uncertainties associated with them. Infrared astronomy is a very powerful tool, with many SETI applications: technology generates heat, and heat is detectable through IR.