This is the Arago spot (Poisson's spot) — explained by diffraction and Huygens' principle.

Explanation:

1. When light waves pass around the circular edge of the obstacle, they are diffracted (bent) into the geometric shadow region — by Huygens' principle, every point on the circular edge acts as a secondary source.
2. At the exact geometric centre of the shadow, all the diffracted waves from the circular edge travel exactly the same distance $\Rightarrow$ they arrive in phase.
3. Constructive interference of all these in-phase waves produces a bright spot at the centre.

Note: Ray optics predicts complete darkness at this point — only wave optics explains the bright spot.