The aim of this talk is to give a pedagogical exposition to why it is
interesting and relevant to study theories of gravity with more than one
metric-like field. These are referred to as bi-metric and multi-metric
theories and always contain a massless spin-2 wave (as in General
Relativity) in addition to a number of massive spin-2 modes that, in
principle, are dark matter candidates. In cosmological set ups, they give
rise to dynamical dark energy that approaches a cosmological constant at
late times. It had long been argued that such theories cannot exist due to
negative energy instabilities. But I will describe multi-metric models
that avoid this problem and have other interesting features.