The image you sent looks to me like the slice is not centered on a region of high density -- more likely, it's centered at the center of the domain, where perhaps the resolution is lower (resulting in the pixels). Can you show the full script you used? And the output of:
or something like that, so we can see where the center is and where the slice is centered? To center on the most dense point I *believe* you can do something like specify the slice center as "max" rather than a number (with the quotes around hte word max).