Stringshard (string-shard, not "strings-hard") is a building material of unknown origin that is frequently used for construction projects spanning entire universes, multiverses, or even larger -verses. It is notable for a series of physics- and logic-defying properties that make it ideal for such a role.
Firstly, it is completely independent of attractive and repulsive forces unless modified. When placed in a certain way, one can "lock" a piece of stringshard into a specific location in space and make it completely immovable. A particularly clever civilization can also configure its gravitational interaction so that all objects experience the same acceleration towards it, regardless of mass or distance, and thus allow for the construction of attached objects that are so large they would normally collapse into black holes.
Secondly, it is almost completely indestructible even at the subatomic level. Stringshard is made of quantum string and quantum foam, carefully bent and then compressed in such a way as to reshape the entire universe around it. It is impossible to destroy stringshard without destabilizing the entire universe it's part of, and to do that you need entire -verses worth of energy compacted down into one attack.
Thirdly, it can be locked to a point in time and then used to pin other objects to that point, erasing them from the entire timeline except for the tiniest, most imperceptible fraction of a second.
Though stringshard normally appears colorless in its base form, it is trivial for an advanced civilization to modify its interaction with photons to give it any imaginable hue.