In 2002 a star in Monoceros flared, and for months afterward telescopes watched the space around it expand faster than light. A glowing shell, swelling outward, crossing years of distance in weeks. It looked exactly like an explosion’s debris flying apart.
Nothing was moving.
The dust was already there — a haze that had hung around the star for ages, cold and dark, reflecting nothing because nothing had lit it. When the star flared, the flash spread out from it at the speed of light, a sphere swelling through the haze. But we never see that sphere. We see only the dust the flash has reached and whose scattered light has then had time to travel all the way back to us. Those are two different clocks. The light that reaches a near grain comes back quickly; the light that reaches a far grain comes back late. So the set of grains we see lit at any one of our instants isn’t the flash-sphere at all. It’s a surface stretched between the star and us — formally a paraboloid, with the star and the Earth at its focus — and that surface sweeps outward through the motionless dust as our seconds pass. The bright shell is the shadow of where two timings happen to agree, moving across grains that never budge.
And the speed is a trick of angle. The dust that looks like it’s three light-years off to the side of the star is really almost six light-years in front of it, tipped toward us. We’re staring down a long tunnel with the star near the back. Light bouncing off the near mouth of that tunnel reaches us almost as soon as the direct flash did, so the lit edge appears to leap across the sky far faster than any real thing is allowed to move. It isn’t outrunning light. It’s just that “to the side” and “toward us” got folded together by the line of sight, and the folding hid the depth.
The piece below is that surface sweeping through a fixed cloud. The grains never move. Each one lights once, as the surface crosses it, and goes dark again behind it.
The star’s outburst was over in weeks. The expansion went on for years: light reaching one motionless grain after another, each grain lit once as the surface passed and dark again behind it, the far ones reporting in long after the near ones, the whole shape of the haze arriving out of order.
Sources: Light echo (Wikipedia); V838 Monocerotis: A Geometric Distance from HST Polarimetric Imaging of its Light Echo (arXiv); What is a light echo? (Astronomy.com).