Odd Radio Circles
Visible only in radio frequencies, Odd radio circles (ORCs)1 are, as the name suggests, faint, ghostly rings of radio emission. Spanning one arcminute in diamater in the sky, these huge oddities span at least a million light-years and are curiously bright around their edges. Only four of them have been found so far, and two of them house a galaxy roughly around their center, theorised to possibly be a progenitor. A lot of different theories have been proposed to explain their existence, including a head-on view of the jets of a radio galaxy to a neutron star merger (read kilonova), but none of them have properly fit the huge size and the location of these things - most supernovas and kilonovas happen in the galactic plane of the Milky Way, and these objects were found in the higher galactic latitudes. Also complicating matters is the fact that these are huge objects, and they could have been propagating and growing over a long period of time, and the progenitor object might be dormant or too faint to detect or observe now. In any case, as of right now, alongside fast radio bursts (FRBs), these are one of the hottest new mysteries in astronomy.