There is a point source of light in an empty universe. What is the minimum number of solid balls (of any size) one must place in space so that any light ray emanating from the light source intersects at least one ball?
To solve this problem, we need to ensure that any light ray emanating from the point source intersects at least one of the solid balls. This means we need to cover the entire surface of a sphere centered at the light source with these solid balls.

1. **Understanding the Problem**:
   - A point source of light emits rays in all directions.
   - We need to place solid balls in such a way that every possible ray from the point source intersects at least one ball.

2. **Geometric Consideration**:
   - The problem can be visualized as covering the surface of a sphere with solid balls.
   - The surface of the sphere represents all possible directions of light rays from the point source.

3. **Minimum Number of Balls**:
   - To cover the entire surface of a sphere, we need to consider the geometric arrangement of the balls.
   - The minimum number of solid balls required to cover the surface of a sphere is related to the concept of spherical covering.

4. **Spherical Covering**:
   - In three-dimensional space, the minimum number of identical solid balls required to cover the surface of a sphere is determined by the arrangement of the balls.
   - The optimal arrangement is such that the balls are placed at the vertices of a regular polyhedron.

5. **Tetrahedron Arrangement**:
   - The simplest regular polyhedron is a tetrahedron, which has 4 vertices.
   - Placing a ball at each vertex of a tetrahedron ensures that the entire surface of the sphere is covered.

6. **Verification**:
   - If we place 4 balls at the vertices of a tetrahedron, any light ray emanating from the point source will intersect at least one of the balls.
   - This is because the tetrahedron's vertices are evenly distributed around the sphere, ensuring complete coverage.

Therefore, the minimum number of solid balls required to ensure that any light ray emanating from the point source intersects at least one ball is 4.

The final answer is $\boxed{4}$.