When all Rigid bodies are created and there are still unlabeled data segments, this is referred to as a gap.
To fill gaps with unlabeled segments:
The red crosshair shows the possible data segments that most likely fill the gap, and rebuild the current marker’s data by joining the two data segments. Keep dragging the Timeline indicator back and forth along the gap to visually analyze what unlabeled segment represents the best fit. Select the unlabeled segment that seems the most appropriate.
You might find an unlabeled segment that is not considered a probable choice, but whose movement fits in the gap. You can force the segment’s data into the current gap by I-clicking the unused segment, which displays as a blue asterisk.