Cast Ray From Middle Of Screen

Vector3 rayOrigin = new Vector3(0.5f, 0.5f, 0f); // center of the screen
float rayLength = 500f;

// actual Ray
Ray ray = Camera.main.ViewportPointToRay(rayOrigin);

// debug Ray
Debug.DrawRay(ray.origin, ray.direction * rayLength, Color.red);

RaycastHit hit;
if (Physics.Raycast(ray, out hit, rayLength))
{
    // our Ray intersected a collider
}