Scientists search for "decaying" dark matter (DDM) because it offers unique signatures like specific X-ray or gamma-ray lines or neutrino signals not seen in normal matter, potentially revealing dark matter's particle nature, mass and interactions, information that could illuminate the universe's structure. DDM is a theoretical model where dark matter particles aren't perfectly stable, but slowly decay over vast cosmic timescales into lighter dark matter particles and/or massless particles, leaving behind gravitational or electromagnetic signals.