That it was the asteroid belt never occurred to me. The field covers a limited area and the size of the sun when Hal looks at it is larger than it would appear from Earth rather than smaller. More like what one would see near Venus. I'd guess it's the rocky remnants of a comet with all the volatiles boiled away. Yeah, it's too dense to be that either, but it's not the asteroid belt.
Well, it CAN'T be the asteroid belt, because it's the other direction. Realistically, it's not the remnants of a comet either.
You know what it is? A writer and a director's poorly thought out idea.