The center of the sphere is the midpoint of the diameter. The midpoint of two points $(x_1, y_1, z_1)$ and $(x_2, y_2, z_2)$ is given by $\left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}, \frac{z_1+z_2}{2}\right)$. So, the center of the sphere is
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