I'm no physicist, but if it's moving through space exploding matter in all directions, wouldn't that form a cylinder?
Not really. Think about what a cylinder is.
Here's a pretty good comparison.

The cylinder shape suggests that the gasses are expanding faster at the ends than the middle. In a vacuum you would expect all the gasses to be expanding at the same rate creating a sphere.
Nope. A cone.
More like a sonic boom:
