Event Horizon is defined as the point at which the gravitational pull becomes so great as to make escape impossible. The dark blue and light blue lines draw the eye to the center which acts like a gravitational pull toward the blue fog lingering in the sphere. The thin white line surrounding the sphere appears to be an event horizon commonly associated with black holes.
This image creates two optical illusions. The first illusion is the 'warping' of the straight lines as they converge on the outer edge of the sphere. The second illusion is the depth and perspective of the lines make the blue fog in the center appear to morph and move within the sphere. If you stare at the blue fog, it will appear as if it hovers within the sphere, and the grey-black smudge in the center of the fog seems to morph into different shapes.