  Nearly every massive galaxy harbors a supermassive black hole (SMBH) in its
nucleus. SMBH masses are millions to billions $M_{\odot}$, and they correlate
with properties of spheroids of their host galaxies. While the SMBH growth
channels, mergers and gas accretion, are well established, their origin remains
uncertain: they could have either emerged from massive "seeds" ($10^5-10^6
M_{\odot}$) formed by direct collapse of gas clouds in the early Universe or
from smaller ($100 M_{\odot}$) black holes, end-products of first stars. The
latter channel would leave behind numerous intermediate mass black holes
(IMBHs, $10^2-10^5 M_{\odot}$). Although many IMBH candidates have been
identified, none is accepted as definitive, thus their very existence is still
debated. Using data mining in wide-field sky surveys and applying dedicated
analysis to archival and follow-up optical spectra, we identified a sample of
305 IMBH candidates having masses $3\times10^4<M_{\mathrm{BH}}<2\times10^5
M_{\odot}$, which reside in galaxy centers and are accreting gas that creates
characteristic signatures of a type-I active galactic nucleus (AGN). We
confirmed the AGN nature of ten sources (including five previously known
objects which validate our method) by detecting the X-ray emission from their
accretion discs, thus defining the first bona fide sample of IMBHs in galactic
nuclei. All IMBH host galaxies possess small bulges and sit on the low-mass
extension of the $M_{\mathrm{BH}}-M_{\mathrm{bulge}}$ scaling relation
suggesting that they must have experienced very few if any major mergers over
their lifetime. The very existence of nuclear IMBHs supports the stellar mass
seed scenario of the massive black hole formation.
