# Testing the nature of dark compact objects: a status report

## TL;DR

This paper reviews the current status of testing the fundamental nature of dark compact objects, exploring whether they are black holes or exotic alternatives, using gravitational-wave and interferometry observations.

## Contribution

It provides a comprehensive overview of the physics, observational evidence, and experimental status of dark compact objects, highlighting recent advances and future prospects.

## Key findings

- Null results support the Kerr black hole paradigm.
- Current observations are consistent with black holes but do not exclude exotic alternatives.
- Future experiments will further test the nature of dark compact objects.

## Abstract

Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.