There is a scientific consensus about the reality of black holes as key ingredients of the physical Universe. Yet, both the inability to
observationally proof the “BH hypothesis” and its challenging and far-reaching theoretical consequences, demand a thorough scrutiny of its alternatives.

In this spirit, a variety of horizonless exotic compact objects (ECOs) have been proposed: the “ECO hypothesis”. Any putative ECO model must overcome theoretical and observational tests to become a contender, either replacing or co-existing, with black holes. In this colloquium we will discuss key challenges for the ECO hypothesis as well as opportunities emerging from a putative lack of Universality of the black hole (and in particular Kerr black hole) hypothesis.