Abstract: Pulsar timing array detection of a gravitational wave background represents a milestone in the field of gravitational wave astronomy. However, the origin of such a signal is still a matter of intense debate: while supermassive black hole binaries formed as a result of galaxy mergers represent the most obvious candidate for the gravitational wave signal, many "new Physics" models can potentially explain at least part of the detected background. In this colloquium I will review the current observational and theoretical status of the field, addressing current issues and future challenges, Moreover, I will discuss how radio observations with the SKAO can contribute to this broader context, and, in particular, I will focus on a possible background detection strategy that involves both pulsar timing array and gravitational wave astrometry.