Your background for the layer is black yes? I assumed that you used an appropriate background color.
If you want to avoid that altogether, than you need to use unbounded scrolling and then manually limit your scroll at the edge of your background image. Something like:
compare:
backgroundImage.X - backgroundImage.Width/2 (assuming your hotspot is in the center) >= ViewportLeft("mainUI")
and
backgroundImage.X + backgroundImage.Width/2 (assuming your hotspot is in the center) <= ViewportRight("mainUI")
and
backgroundImage.Y - backgroundImage.Height/2 (assuming your hotspot is in the center) >= ViewportTop("mainUI")
and
backgroundImage.Y + backgroundImage.Height/2 (assuming your hotspot is in the center) <= ViewportBottom("mainUI")
and if you want to keep your critter in the center of the screen, then you have to use a suitable layer background color and just suffer the sidebars... you can't have your nice scrolling and eat it too.