The recent discovery of super-massive black holes ( SMBHs ) in high-mass ultra-compact dwarf galaxies ( UCDs ) suggests that at least some UCDs are the nuclear star clusters of stripped galaxies .
In this paper we present a new method to estimate how many UCDs host an SMBH and thus are stripped galaxy nuclei .
We revisit the dynamical mass measurements that suggest many UCDs have more mass than expected from stellar population estimates , which observations have shown is due to the presence of an SMBH .
We revise the stellar population mass estimates using a new empirical relation between the mass-to-light ratio ( M/L ) and metallicity to predict which UCDs most likely host an SMBH .
We calculate the fraction of UCDs that host SMBHs across their entire luminosity range for the first time .
We then apply the SMBH occupation fraction to the observed luminosity function of UCDs and estimate that in the Fornax and Virgo cluster alone there should be 69 ^ { +32 } _ { -25 } stripped nuclei with SMBHs .
This analysis shows that stripped nuclei are almost as common in clusters as present-day galaxy nuclei .
We estimate the SMBH number density caused by stripped nuclei to be 2 - 8 \times 10 ^ { -3 } Mpc ^ { -3 } , which represents a significant fraction ( 8-32 % ) of the SMBH density in the local Universe .
These SMBHs hidden in stripped nuclei increase expected event rates for tidal disruption events and SMBH-SMBH and SMBH-BH mergers .
The existence of numerous stripped nuclei with SMBHs are a direct consequence of hierarchical galaxy formation , but until now their impact on the SMBH density had not been quantified .