The death of the universe.
The bright signatures of billions of distant galaxies have long since disappeared. Those that remain visible are spinning perpendicular to us and appear as angry red eyes. Viewed edge on, they are all but invisible.
The pitch black pupils gave no hint that they were once a densely populated galactic nucleus with giant stars, luminous gas clouds and stellar nurseries.
The visible light in the iris comes from the remaining stars. The red giants, the white dwarfs. Their fuel all but exhausted, the fusion reaction continues in fits and starts. Nothing like the white hot efficiency of a newborn star, they cooled and expanded to hundreds of times their size. They had the hue of smoldering embers.
The reality of it was that these were no longer active living galaxies. The Hydrogen needed to give birth to new stars had been exhausted long ago. The only stars that remained were dying of old age or already dead.
The final phase of a galactic destiny was to become an accretion disk around the Super Massive Black Hole at their centers. The object that triggered the formations of these galaxies in the first place, was now the final engine of their destruction.
Through the galactic lifespan, there was a super massive black hole of 10,000 or so stellar masses resident in the galactic nucleus yet hidden by the surrounding bulge of the galactic core.
These had grown, now possessing the mass of over 18 billion stars, with a gravity well so powerful galaxies hundreds of light years apart are drawn towards each other. Super Massive Black Holes have a poorly defined event horizon. A much larger mass, but taking up proportionately a much larger volume of space than Stellar and Intermediate sized Black holes.
The closer the Mass/Volume ratio, the less violent the tidal forces at the event horizon.
Smaller, Stellar Mass Black Holes of 3-6 Solar masses with the volume of a grapefruit, had an event horizon so abrupt that planets observed crossing over were described as being stretched then dissolving like a Styrofoam ball against a belt sander. The unimaginably violent tidal forces can increase gravity exponentially from one foot to the next, causing spaghettification then obliteration, literally tearing matter apart at the sub-atomic level. Yet, for all their power, they remain relatively local phenomena.
Aside from the odd star system unlucky enough to drift too close, these Galactic black holes and the spent remnants of a billions star systems have reached an uneasy truce.
That is not to say they don't still feed.
Once a rare event, galactic collisions are constantly occurring. Drawn together by massive gravity wells, the results are a cataclysmic. A galactic merger.
Even now, as two galaxies draw nearer, one twice the size of the other, they begin to twist and distort as the previously unswallowed matter of one galactic accretion disk is drawn into the SMBH of the colliding galaxy.
The mass of billions of star systems will be absorbed. Billions more will be slingshot into deep space. These are the ones with a stay of execution.
As the Event Horizons of the two Super Massive Black Holes draw close, their rate of closure increases exponentially, they intersect, overlap, become one. The energy released by the shearing tidal forces of the two giants as the larger SMBH absorbs the smaller one will spike along with the gravity well which immediately begins drawing in the remains of the two accretion disks not thrown into space.
The reprieve of those star systems to have escaped is temporary. They will begin to orbit the winner from an even greater distance, eventually dispersing more evenly. A new accretion disk, waiting for the next hostile takeover.
If the colliding SMB's had been of more or less equal mass, the may have retained their individuality and begun spinning around each other. A Binary System.
More and more Binary and Trinary Super Massive Black Hole Systems exist.
As more collisions occur, the once expanding mass of the universe is slowly, inexorably, reconstituting. Over countless millennia, their number will dwindle as the survivors grow in volume.
The End of Time...
The death of the Universe will be precipitated by the merger of the last Galactic Black Hole systems. As the last remaining matter in the universe is drawn in, the event horizon, now light years in diameter, collapses in on itself.
As the collapse continues, all the matter in the universe takes up the space of a basketball, a softball, a golf ball, a pin head. Time slows, stands still, stops.
A singularity. Nothing. Time and space are meaningless.
The unimaginable gravitation forces at work have created heavy elements. So dense they can only exist at the core of a black hole, so unstable they create a growing fusion reaction whose energies rival the gravity well keeping them captive. Building, doubling, tripling, exponentiating until they can no longer be contained.
Gravity is defeated.
A Black Super Nova.
A Big Bang
The cycle begins again.
The dawn of time.