It's something called a protoplanetary disk likely a solar system just forming around a very young star. It's mostly dust and other material in the shape of a thin disk seen edge-on. It is denser than the surrounding nebula and blocks the light from behind, so the disk appears dark against the glowing gas of the nebula. It's something like what our solar system might have looked like billions of years ago when the Sun was just forming. Many of these objects have been observed by Hubble in the Orion Nebula.
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