The accreting black hole HLX-1 has been hailed as the first observational proof of the link between dwarf satellite galaxies,massive star clusters and intermediate-mass black holes formed inside them.However,a consistent model of this puzzling system still eludes us.X-ray luminosity and spectral properties strongly support the intermediate-mass black hole scenario(a few times 104 solar masses),but the outburst sequence is more similar to those of stellar-mass black holes.The kinematics derived from optical spectroscopy is consistent with its location in a disrupted satellite galaxy,but optical flux variability suggests that most of the blue/UV emission comes from an accretion disk rather than a star cluster.The lack of any other known source with the same properties in the local universe makes this object even more intriguing.