Normal matter (baryons and electrons) both interact strongly with electromagnetic radiation. As a result, at early times before the formation of the CMB, when the radiation density is so high that it dominates the dynamics of the Universe, the normal matter is not able to begin collapsing into structures. When finally the radiation density drops low enough, the baryons can begin to collapse, but at that point it is too late. The expansion of the Universe has proceeded to a point that there is not enough time to form the galaxies and clusters of galaxies that we see populating our Universe today. However, if some amount of special matter is added that responds to gravity but not to radiation, then it can begin to collapse early on. It can “seed” the structures, and the baryons then fall quickly into the gravity of the dense regions created by this special matter. Of course, the type of matter that responds gravitationally but not electromagnetically has exactly the properties of the dark matter that we observe via its effects on galaxy rotation curves and the motions of gas and galaxies in galaxy clusters. So cosmologists find that they cannot get their model Universes to resemble our own unless they add a bit of dark matter to them, and the best fit to observations occurs when the dark matter accounts for a quarter or so of the total amount of the critical density.
The size and spacing of the structures in the Universe are altered slightly by the presence of dark energy since it tends to increase the expansion rate. The increased expansion tends to inhibit the formation of structures. The gravitational effect from matter (baryon and dark) is attractive and encourages the creation of clumps, but the gravitational interaction from the dark energy has the opposite effect. The influence of dark energy is proportionately larger for smaller matter concentrations because their gravitational attraction is weaker. As a result, the dark energy tends to suppress the formation of small objects, while larger objects (large galaxies and clusters of galaxies) are affected relatively less. By comparing the distribution of structures in the real Universe to those in computational models astronomers have determined that the dark energy accounts for about 70% of the critical density. The research also suggests that its strength (though not its total amount) has likely been constant over time, an important clue to the nature of the dark energy itself.