The substance a phage leaves outside its host cell is its empty capsid.

Here's why:

* Phages inject their genetic material (DNA or RNA) into host cells. The capsid, which is the protein shell that protects the genetic material, stays outside.

* After the phage replicates inside the host cell, new phages are assembled. These new phages burst out of the cell, leaving behind empty capsids of the original phage.

* The empty capsid can still be detected. Researchers can use techniques like electron microscopy to see these empty capsids, providing evidence of phage activity.

Let me know if you have any other questions!