Here's a similar effect made in Unity (https://mmxxxviii.itch.io/volumetric-blackhole https://mmxxxviii.itch.io/volumetric-blackhole), with a breakdown video ab

Here's a similar effect made in Unity (https://mmxxxviii.itch.io/volumetric-blackhole https://mmxxxviii.itch.io/volumetric-blackhole), with a breakdown video about how it was made, for those of you that are curious. It's possible this one uses a completely different technique, but it also appears raymarched to be raymarched.