Starship Asterisk*

Does anyone have any idea what would cause the comet like item near picture center?

I'm not sure I understand since the whole structure appears somewhat comet-like. I suppose perhaps you are referring to a smaller white streak aligned about 30 degrees from horizontal just below the middle of the frame?

My first thought was an x-ray jet from a black hole, but in retrospect I think an x-ray jet would have to be pointed roughly at us to be directly detected. However, it may also be an x-ray jet interacting with interstellar gas. Since it appears to be nearly white, it is presumably a mix of high, medium, and low energy x-rays (shown in blue, green, and red). Perhaps it is a star cluster shadowed by closer features in the galactic core, or perhaps it's a more distant x-ray source that has been gravitationally lensed.

http://antwrp.gsfc.nasa.gov/apod/ap060810.html
The streak in the middle does indeed look comet like; but given the distande it is probably larger than our solar system.
Orin

I seem to recall those structures being called 'filaments' (though I could be misremembering), and that astrophysicists weren't entirely sure what the heck was causing them.

http://antwrp.gsfc.nasa.gov/apod/ap050403.html

Qev
This one is what I believe you are refering to. And I believe you're right about the filaments