There is lots of unexplored territory in molecular spectroscopy. The ALMA Telescope[0]—primarily operating at millimeter and sub-millimeter wavelengths, where the bulk of molecular transitions are—is likely to discover many spectral lines which have not been classified. Estimates suggest that 50–80% of the lines seen in some observations will correspond to unknown transitions of known molecules or transitions from previously unknown molecules. But the energies of molecular (rotational) transitions are generally far too low for an unknown molecular species to be a viable explanation for a spectral line in the X-rays.

[0] https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array