docs-matrix-spec/content/rooms/fragments/v1-event-ids.md

An event has exactly one event ID. Event IDs in this room version have the format:

$opaque_id:domain

where domain is the server name of the homeserver which created the room, and opaque_id is a locally-unique string.

The domain is used only for namespacing to avoid the risk of clashes of identifiers between different homeservers. There is no implication that the room or event in question is still available at the corresponding homeserver.