Curious as to what this is..

Dear ESASky User,

Apologies for the delay in replying (Summer holidays). 

I'm not sure what this is, it could be instrumental, perhaps it's some instrumental affect that has been removed from the image by a processing pipeline and now appears dark? The colour image you see in the background of ESASky comes from the second Digitized Sky Survey (DSS2), which is a digitized version of several photographic surveys of the sky, produced by the Space Telescope Science Institute between 1983 and 2006. Or perhaps you are correct, and it is a text mark, written by hand on to the original photographic plate. I'd suggest asking the Space Telescope Science Institute who created DSS2, they can be contacted here via their archive helpdesk: https://stsci.service-now.com/mast

The colour background (HiPS) in ESASky was created by the Strasboug astronomical Data Center (CDS) by combining the three DSS2 filters in the red, blue and infrared. I downloaded a cutout of the three original images and you can see that the affect only appears in the red DSS2 image (downloaded from here https://archive.eso.org/dss/dss) :

I hope this helps and sorry I can't give a definitive answer!

Kind regards,

Deborah