Brad, the bubble is just the fact that the realtime preview is bubble like is due to the fact that it is a low res representation (we are talking a lot of data in one of these images, 60gb Fits files in my case) made worse still by jpg artefacts for posting here. The changes made do effect the whole image. I may well have a darker background than you due to use of a light pollution filter or less light pollution. Just means you wont be able to stretch quite as far. The background should NOT be black. This whole area of Orion is filled with faint nebulosity so even at a very dark site, the background will not be black, probably a faint red but dark grey is good.
You can zoom in (yes the 1:1) and make changes, it is very senstive at 1:1. Remember the LH slider affects the black point, dont move it further than the bottom of the steep part of the histogram. Then move the mid tone carat to the left to somewhere just inside the right hand side of the histogram.