The cosmic triangle is introduced as a method to represent the past, present, and future status of the universe. The current location within the cosmic triangle is determined by three key questions: the amount of matter in the universe, whether the expansion rate is slowing down or speeding up, and whether the universe is flat. Recent observations suggest that the universe is lightweight (matter density about one-third of the critical value), accelerating, and flat. The acceleration implies the existence of cosmic dark energy, which overcomes the gravitational self-attraction of matter and causes the expansion to speed up. The paper reviews various methods to determine these parameters, including the mass-to-light method, baryon fraction method, cluster abundance and evolution, mass power spectrum, supernovae observations, gravitational lensing statistics, and cosmic microwave background (CMB) anisotropy measurements. These methods collectively confirm a low-mass-density universe, an accelerating expansion, and a flat universe. The paper also discusses the implications of these findings for the future fate of the universe and the need to understand the fundamental physics underlying dark energy.The cosmic triangle is introduced as a method to represent the past, present, and future status of the universe. The current location within the cosmic triangle is determined by three key questions: the amount of matter in the universe, whether the expansion rate is slowing down or speeding up, and whether the universe is flat. Recent observations suggest that the universe is lightweight (matter density about one-third of the critical value), accelerating, and flat. The acceleration implies the existence of cosmic dark energy, which overcomes the gravitational self-attraction of matter and causes the expansion to speed up. The paper reviews various methods to determine these parameters, including the mass-to-light method, baryon fraction method, cluster abundance and evolution, mass power spectrum, supernovae observations, gravitational lensing statistics, and cosmic microwave background (CMB) anisotropy measurements. These methods collectively confirm a low-mass-density universe, an accelerating expansion, and a flat universe. The paper also discusses the implications of these findings for the future fate of the universe and the need to understand the fundamental physics underlying dark energy.