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Stars form mostly in dense, molecular gas, easily observable in low-J rotational transitions of CO. The lower density, diffuse gas out of which molecular clouds are forming is considerably more difficult to observe. However, it is crucial to assess this 'CO-dark/poor' gas since it may contain a large fraction of the total mass of gas, and thus dominating the global reservoir of kinetic energy of the clouds.It may also play an important dynamic role in the molecular cloud formation process. Thanks to new high sensitivity spectrally resolved observations of the [CII] 158 micron line within the FEEDBACK program on SOFIA, we present for the first time compelling evidences that [CII] traces regions of CO-poor gas and interactions between atomic/molecular clouds in the Cygnus X region at very different radial velocities. We derive a density of ~100 cm^-3 and a relative velocity difference up to 20 km/s for the gas. We dub this interaction 'soft collisions' and suggest that they could be a frequent way how dense structures are forming with the [CII] 158 micron line as the best tracer for witnessing this assembling process of molecular clouds.