...cluster...? ...rather than just a planetary nebula in our own galaxy, that we are seeing this cluster *through*? This is supposed to be evidence of dark matter; the lens effect. We don't see any other galaxy clusters with big luminous clouds around them. But we sure do see a lot of planetary nebulae resembling this structure. So can anyone tell me how that possibility was ruled out?

http://hubblesite.org/gallery/album/galaxy_collection/pr2007017a/
Akash, I suppose I'll have to do that if I don't get the quick & easy, down & dirty answer here. It's a Hubble image, but I'd have to do some research and no doubt wait some time just to find out who cl***ified it as such.
Charlie, thank you. Yes, I have seen that article, but I am still looking for an explanation of why this thing is not a local planetary nebula. I guess I will have to track down the astronomer(s) who made this claim.
Manny, I'm glad I'm not the only one who has that reaction. So Prof, they superimposed a computer generated image over the cluster. This image was generated by data that they say deviates from what should be expected. So the computer generates the halo it was essentially instructed to generate. At least I know now that it isn't a planetary nebula. That would have been a mistake. Now I wonder if it is fraud. No, I'm not charging that, yet. I still have a lot to learn about this.
Charles, I was going to say that I should look up the red shift of this halo next. That would be reasonable, but Prof has told us it isn't even a visual image!
Sparkle, lots of good stuff, there. Especially the "who" part. The URL would have been nice.
Okay, I've spent some time studying a large, hi-res view of this image. There are clear variations in the density of this superimposed halo which don't correspond with the distribution of galaxies at all. Nor does there appear to be any evidence of lensing of these visible galaxies that I can see.
Okay, Prof, you're working on me. You do it all the time? Repeatable? Blind consensus? All right, then, I need to do some more studying. Next, I'm going to try to learn how lensing is determined from the photo. I got a book for Christmas that reviews all this stuff, so I'll dig into that and give the forum a break for awhile.
Thanks for all the information and fine effort and time most of you took. Now I should go read the book.