From Latour's Paris, the Invisible City

When there's a lack of techniques, when by chance a strike or breakdown deprives us of a means of communication or transport, everyone learns, walking and talking, that the social world is indeed flat, that it has to be composed piece by piece, staircase by staircase, concierge by concierge. When riots are rumbling no one believes that there is a Society, constantly present, with little individuals living in it. From every bridge insurrection can emerge, a new totality, a new regime, marching through Paris, offered to Parisians. Switching from the real to the virtual Paris means finding the road to these potential totalities, these scattered virtualities, yes, these former virtues (the word "virtual", don't forget, also derives from virtus, the favourite world of the ancient Romans), from this plasma —the word meaning a fine layer of clay that Prometheus was said to have used to model Pandora. (63)

From Graham Harman's Prince of Networks 

The main discussion of plasma comes at the end of the book. As Latour puts it, ‘no understanding of the social can be provided if you don’t turn your attention to another range of unformatted phenomena […]. I call this background plasma, namely that which is not yet formatted, not yet measured, not yet socialized, not yet engaged in metrological chains, and not yet covered, surveyed, mobilized, or subjectified’ (RS, pp. 243-4). He even estimates the size of this background plasma, just as astronomers indirectly size up the amount of dark matter: Latour says that if the social world of networks were the size of the London Underground, plasma would fill the remaining space of London. A truly vast space of unsocialized material! The plasma is ‘in between and not made of social stuff. It is not hidden, simply unknown [… like] a vast hinterland […, like] the countryside for an urban dweller […, like] the missing masses for a cosmologist trying to balance out the weight of the universe’ (RS, p. 244).

In these startling pages, Latour amplifies the realism of Irreductions with a beautiful image: ‘Hermeneutics is not a privilege of humans but, so to speak, a property of the world itself. The world is not a solid continent of facts sprinkled by a few lakes of uncertainties, but a vast ocean of uncertainties speckled by a few islands of calibrated and stabilized forms’ (RS, p. 245). Latour’s plasma is not a mere annoying remainder of academic bookkeeping, but is held responsible for all the change and movement we know. Above all, there is a stunning passage that one would never find in Heidegger:

Why do fierce armies disappear in a week? Why do whole empires like the Soviet one vanish in a few months? Why do companies who cover the whole world go bankrupt after their next quarterly report? Why do the same companies, in less than two semesters, jump from being deep in the red to showing a massive profit? Why is it that quiet citizens turn into revolutionary crowds or that grim mass rallies break down into a joyous crowd of free citizens? Why is it that some dull individual is suddenly moved into action by an obscure piece of news? Why is it that such a stale academic musician is suddenly seized by the most daring rhythms? Generals, editorialists, managers, observers, moralists often say that those sudden changes have a soft, impalpable liquid quality about them. That’s exactly the etymology of plasma (RS, p. 245).

These things happen because the articulated social world of relations leaves so much unarticulated: monsters and angels seep from the plasma, like rats and pigeons into the Underground. In his concluding summary of plasma, Latour even uses a term once banned from his personal lexicon: ‘To every action I have described so far, you have to add an immense repertoire of missing masses [… And there] exists a reserve, a reserve army, an immense territory […] for every formatted, localized, continuous, accountable action to be carried out in’ (RS, p. 245, emphasis modified). Here Latour seems to understand that there is a problem with explaining how completely formatted actors could ever change their format. As his new thoughts indicate, the only possible solution is that actors are not fully formatted by alliances after all. Some reserve or reservoir must explain the sudden changes mentioned, and more gradual changes as well. To escape relationism means to establish a metaphysics of the plasma or missing mass to which Latour refers. Only one note of caution is needed: there is no good reason to agree with Latour that the plasma has no format, since this would imply that all format must come from relations. The plasma might still be formatted by other means than those of alliance, though he never considers the possibility. Formatting and networks are viewed as equivalent, and at best Latour verges on acknowledging a single plasma-in-itself, not a plurality of distinct entities withdrawn from all relation. (133-34)