  In this Essay we address several fundamental issues in cosmology: What is the
nature of dark energy and dark matter? Why is the dark sector so different from
ordinary matter? Why is the effective cosmological constant non-zero but so
incredibly small? What is the reason behind the emergence of a critical
acceleration parameter of magnitude $10^{-8} cm/sec^2$ in galactic dynamics? We
suggest that the holographic principle is the linchpin in a unified scheme to
understand these various issues.
