Right at the center of this image, there's a dim orange dot. Somewhere around that dot, one of the stars in a binary system once went supernova, creating the vast red gaseous remnant you see all around it.
The problem is that no one can find the other star in that binary system, the one that should have been left over when the star went supernova. And that's bad news, because this is a Type Ia supernova, a particular type of cosmic explosion that happens when a white dwarf explodes. Because white dwarfs always have just about the same mass when they explode in these supernovas, they always have the same luminosity, which makes Type Ia supernovas incredibly useful in calculating the distances of faraway galaxies. Indeed, it was measurements of just these supernovas that helped reveal the existence of dark energy.
But there's a lot more we could understand about Type Ia supernovas — and, by extension, the precise nature of deeper cosmic mysteries like dark energy — if we could find the companion star left over from this explosion. That's why its apparent absence is so frustrating. As NASA explains:
At the center of this supernova remnant should be the companion star to the star that blew up. Identifying this star is important for understanding just how Type Ia supernova detonate, which in turn could lead to a better understanding of why the brightness of such explosions are so predictable, which in turn is key to calibrating the entire nature of our universe. The trouble is that even a careful inspection of the center of SNR 0509-67.5 has not found any star at all. This indicates that the companion is intrinsically very faint — much more faint that many types of bright giant stars that had been previous candidates. In fact, the implication is that the companion star might have to be a faint white dwarf, similar to — but less massive than — the star that detonated.