The Hubble diagram continues to remain one of the most important graphical representations in the realm of astronomy and cosmology right from its genesis that depicts the velocity-distance relation for the receding large-scale structures within the Universe; it is the diagram that helps us to understand the Universe’s expansion. In this paper I introduce the molecular expansion model in order to explain the expansion of the Universe. The molecular expansion model considers the large-scale structures as gas molecules undergoing free expansion into the vacuum. Large-scale structures being ensemble of atoms must behave like molecules possessing finite amount of energy. Since metric expansion of space cannot be tested practically and can only be observed indirectly due to the presence of observable entities, therefore, instead of considering the metaphysics of expanding space, the paper emphasizes upon the actual recession of large-scale structures as the most natural reason to explain the observed expansion. I show in this paper that the linear velocity-distance relation or the Hubble diagram is actually a natural and a characteristic feature of different gas molecules undergoing free expansion into the vacuum at the same time. Different gas molecules naturally have different velocities, and, molecules being natural entities provide a natural and a scientifically-viable explanation better than metaphysics. The study conducted in this paper finds the recessional behaviour of large-scale structures to be consistent with the recessional behaviour of molecules. The free expansion of different gas molecules into the vacuum of the Universe is found to be homogeneous, isotropic and in agreement with the Copernican principle. Redshift-distance relationship has been plotted for 580 type Ia supernovae from the Supernova Cosmology Project, 7 additional high redshift type Ia supernovae discovered through the Advanced Camera for Surveys on the Hubble Space Telescope from the Great Observatories Origins Deep Survey Treasury program, and 1 additional very high redshift type Ia supernova discovered with Wide Field and Planetary Camera 2 on the Hubble Space Telescope. Redshift-distance relationship for these 588 type Ia supernovae has been analysed and the reason for the deviation of the Hubble diagram from linearity at high redshifts has been explained without acceleration by introducing the concept of differential molecular expansion.