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Bubble universes and traversable wormholes in general relativity can berealized as two sides of the same concept. To exemplify, we find, display, andstudy in a unified manner a Minkowski-Minkowski closed universe and aMinkowski-Minkowski traversable wormhole. By joining two 3-dimensional flatballs along a thin shell two-sphere of matter, i.e., a spherical domain wall,into a single spacetime one gets a Minkowski-Minkowski static closed universe,i.e., a bubble universe. By joining two 3-dimensional complements of flat ballsalong a thin shell two-sphere of matter, i.e., a spherical throat, into asingle spacetime one gets a Minkowski-Minkowski static open universe which is atraversable wormhole. Thus, Minkowski-Minkowski bubble universes and wormholescan be seen as complementary. It is also striking that these two spacetimeshave resemblances with two well-known static universes. The Minkowski-Minkowskistatic closed universe resembles the Einstein universe, a static closedspherical universe homogeneously filled with dust matter and with acosmological constant. The Minkowski-Minkowski static open universe resemblesthe Friedmann static universe, a static open hyperbolic universe homogeneouslyfilled with negative energy density dust and with a negative cosmological, auniverse with two disjoint branes that can be considered a failed wormhole. Inthis light, the Einstein and Friedmann universes are also two sides of the sameconcept. A linear stability analysis for all these spacetimes is performed. Thecomplementarity between bubble universes and traversable wormholes, that existsfor these static spacetimes, can be can carried out for dynamical spacetimes,indicating that such a complementarity is general. The study suggests thatbubble universes and traversable wormholes can be seen as coming out of thesame concept, and thus, if ones exist the others should also exist.