Invisible Black Matter (インビジブルブラックマター Inbijiburu Burakku Matā?) abbreviated as IBM, are the main source of an Ajin's powers. IBM cannot be seen by people who are not Ajins. As theorized by Ikuya Ogura, IBM is what allows the Ajin to replace any lost body tissues or nutrients as demonstrated when an IBM oozes from the body of an Ajin when they're fatally wounded or when they're manifesting their Black Ghost. The origins of IBMs are still unknown as are their composition; IBM's are made up of intangible unknown matter that originates from an Ajin's body. Each Ajin has a different amount of IBM but nonetheless they all regenerate at the same rate - the only difference that the IBM concentration makes is how frequent they can manifest their Black Ghosts; Kei's IBM concentration is quite high compared to the average Ajin and thus he's able to manifest his Black Ghost more frequently and for longer periods of time with the maxium so far being 30 minutes compared to the average 5.
Ajins also possess the ability to manifest themselves into Black Ghosts. Black Ghosts and IBM are different in the sense that all Ajins possess IBM, but not all of them can manifest a Black Ghost. (Nakano is one such person who can't manifest a Black Ghost.) These Ghosts are made up of IBM and normally have a humanoid form but can have inhuman characteristics, standing at about over two meters tall. Each Ajin can manifest a unique Black Ghost. Ghosts are usually used for combat purposes, but they can also be utilized in other ways if their hosts know how to control them properly. They only have one weak point: their heads. Only a fatal blow to the head by another Ghost can destroy a Ghost. The connection between an Ajin and their Ghosts can also be hindered by water; analogous to how a cell phone's signal can be disrupted when there's heavy rain. If an Ajin and their Ghost's 'connection' are severed, the Ghost will vanish eventually.
Ajins can manifest Black Ghosts when they're aware that they're Ajin (usually when they die), although Nakano hasn't manifested his Ghost despite dying multiple times already. When 2 Black Ghosts' heads make contact, the memories of the Ajins that own them also 'collide', resulting in a brief moment where their respective hosts are able to see a memory from the other Ajin. During this collision they also trade their mental states such as fear. This happened first with Kei Nagai and Satō's Ghosts.
These Black Ghosts also seem to have a will of their own and are shown to be quite intelligent. Black Ghosts often mimic the 'core' personality of their host, as seen in Nagai and Sato's cases. Black Ghosts can also mimic their host's speech as demonstrated by Sato and Kei. However, there are also cases where Black Ghosts are left to their own will due to their host's negligence to control them or failure to do so. One case included a farmer that left their Ajin to it's own will, and it eventually taught itself things such as farming or riding a tractor. If left alone for too long, it will be much harder to control them, as in Kei's case, who didn't know he had a Black Ghost for almost his entire life as he manifested it before he even died without him knowing how.
It is interesting to note that Satō, Tanaka, Izumi, Okuyama, Takahashi and Kotobuki are able to control their IBM's completely. Kei's IBM simply does whatever it wants, even completely ignoring Kei. While other Ajins are able to command their Ghost properly and can share their Ghost's field of vision and command it to attack targets, Kei is unable to do so. His Ghost is reckless, and often attacks anyone in its immediate vicinity.
It is also possible for Black Ghosts to have wings, as in Takeshi Kotobuki's case. It is unknown whether or not this is possible for other Ajins as well due to each Ajin's Ghost being unique. Ikuya also suggests that Black Ghosts have the capability to change if their user are able to change their personalities. However, it is yet to be seen if it's actually possible.
If an Ajin dies when experiencing a one in a life time heightening of emotions, they are able to cause a "flood" of IBM's and cause havoc by summoning multiple Black Ghosts that they can't control. 10-20 IBMs are generated depending on the density of the Black Ghost of the user. However, this is a very rare occurrence; the only known cases of a Flood occurring in Japan happened with Nakamura's and Kei in the anime.
In some cases of a flood nearby people who had recently died may be affected. During the Shinya Nakamura incident, Yūsuke died due to cardiac arrest despite being shot in the chest. The gunshots had healed but he couldn't reach the point of becoming an Ajin. In other words he was on the threshold. It is unknown if Yūsuke would have come back if he received CPR. A scientist from the US believes that this phenomenon is a hint to how Ajins are born but Ikuya Ogura argues that this is due to how strong the Ajin's emotions are at the moment of the flood.
The IBMs created during a flood continuously act according to the emotion that triggered it's appearance. There was a case where a dutch gold medalist fell over and died right after reaching the finish line. The joy of victory combined with his revival caused a flood where the IBMs were rejoicing. The gold medalist attempted to order them to leave but they refused to listen. In Shinya Nakamura's case it was malice that triggered the IBMs and if any pedestrians were nearby they would have also been killed.
- The concept and uses of an IBM are quite similar to stands of the Jojo's Bizzare Adventure series as both are ghost like creatures who help their users can only be seen by a specific type of people (in Jojo being stand users.)