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Dark nebulae serve as the crucial cradles of star formation in the cosmos. Think of them as colossal, opaque cosmic clouds brimming with gas and dust. Their opaqueness is due to the dense composition of dust particles, which effectively absorb and scatter light, cloaking the regions within in darkness. This obscuration of light creates the ideal chilly nursery needed for the birthing of stars.

Since these nebulae are cold, the atomic and molecular motion within is relatively sluggish. This reduced kinetic energy allows gravitational forces to overcome the internal pressure that would otherwise disperse the gas and dust, setting the stage for the formation of new stars, or protostars, as they are known in their infancy. Dark nebulae, with their cool and serene environment, are therefore indispensable in the cosmic saga of star birth.

The interstellar medium (ISM) is like the canvas of the universe – a complex blend of gases, particles, and cosmic dust interspersed between the stars. This 'space-filling' substance makes up a significant portion of the material in a galaxy and provides the building blocks for star formation. It is composed primarily of hydrogen and helium gas, with a sprinkling of other heavier elements and molecules.

The diversity in the ISM's phases, from hot ionized to cool molecular clouds, sets the tone for various astronomical phenomena. For aspiring protostars, it is the cooler, denser parts of the ISM that hold significance. In these parts, atoms and molecules are capable of clumping together under gravity's relentless pull. With conditions just right, pockets within the ISM can begin to coalesce and, through the fateful process of gravitational collapse, give rise to nascent stars.

Gravitational collapse is akin to the cosmic 'tug-of-war' that decides the fate of a potential star. In this delicate balance between external pressure and the relentless pull of gravity, gravity eventually prevails under the right conditions, leading to a protostar's creation. This process initiates when the internal pressure, provided by gas and radiation, is no longer sufficient to counteract the force of gravity pulling the material inward.

For a protostar, the journey begins in the core of a dark nebula, where material accumulates and density increases until a critical point is reached. The ensuing collapse causes the core to heat up, transforming potential energy into kinetic energy. Eventually, if the collapsing cloud core reaches sufficiently high temperature and pressure, nuclear fusion ignites, and a new star begins to shine. This process underscores the importance of the delicate interplay between gravity and temperature in star formation.