All the atoms and light in the universe together make up less than 5% of the total contents of the cosmos. The rest is composed of mysterious dark matter and dark energy. Dark matter (27%) makes up most of the mass of galaxies. Dark energy (68%) is the mysterious influence driving the expansions of the universe.
Looking up at the night sky, it seems as though the stars and galaxies are spread out in a more or less random fashion. This, however, isn’t really the case. The universe isn’t a random jumble of objects; it has a structure composed of interconnecting filaments of galaxies and gas that form a Cosmic Web.
Think of this Forest as the Universe, the Trees as Galaxies, the Leaves as Stars, the Soil as Dark matter, and the Nutrients in the soil that fuel the growth and expansion of the forest as Dark energy. As long as there is soil and nutrients to fuel new growth the forest would expand exponentially forever.