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For those that need it spelled out, Cygnus is a pressurized module that carries experiments internally and can only subject them to zero g.   Dreamchaser does the same thing with the additional ability to return back to earth.  X-37 carries experiments in an unpressurized bay that once on orbit is exposed to space (vacuum and viewing).  Much like any other spacecraft bus (Starlink, A2100, BSS 702, etc) but with the ability to return.