Gravitational lensing.

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A ring

In 1919 astronomers measured stars seen near the Sun during an eclipse and found they had shifted exactly as Einstein predicted: the Sun’s gravity had bent their light. A whole galaxy, a trillion times heavier than the Sun, is a gravitational lens. The Scene has a giant elliptical galaxy, and a spiral twice as far away almost exactly behind it.

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The ring’s size

The ring’s radius, the Einstein radius, depends on the lens’s mass and the distances:

$\theta_E = \sqrt{\frac{4GM}{c^2} \cdot \frac{D_{LS}}{D_L D_S}}$$\theta_E =$$\sqrt{\frac{4GM}{c^2} \cdot \frac{D_{LS}}{D_L D_S}}$⁠

$D_L$⁠ and $D_S$⁠ are the distances to the lens and to the source, and $D_{LS}$⁠ the distance between them. Lens mass is locked at Galaxy group, $4$⁠ trillion solar masses.

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Arcs

A complete ring needs almost perfect alignment. Offset from the line moves the distant galaxy sideways, in arcseconds.

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A cross

Real galaxies are not perfectly round. Flattening of the lens makes it oval, and changes how many images there are.

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Weighing what we cannot see

Show where it really is is halfway: lines follow the light from points on the rim of the distant galaxy, bent at the lens and sent on to us. Turn the view to see them from the side. Go on to Where it really is: the bending relaxes, and the arcs gather back into one small spiral.