Interstellar space is inconceivably vast. The nearest star group is the triple system of Alpha Centauri. Currently, the red dwarf companion named Proxima Centauri is slightly closer to the earth at 4.3 light years away. While that doesn’t sound particularly far, consider things on a more comprehensible scale: imagine the sun as a period on this page… the Alpha Centauri system would be three other periods, with Alpha Centauri centered at 13.5 kilometers away.
Alpha Centauri A and B are slightly larger and slightly smaller than our sun respectively. Proxima centauri is an M type red dwarf and contains only about 12% of our Sun’s mass. The majority of our nearby stars are similar in size and luminosity to Proxima Centauri. In fact Red Dwarfs are the most common type of star in the universe. These stars are small, dim, prone to tantrums and any planets close enough to be inside the habitable zone are almost always tidally locked (keeping the same face towards the star).. So, while abundant, these stars do not appear to be very promising endpoints for interstellar journeys. Unfortunately, after Alpha Centauri A & B, the next worthwhile star is almost twice as far. There are only a small handful of sun-like stars within 15 light years.
The nearest discovered exoplanet appears to reside around Alpha Centauri B but boasts a temperature of 1200 Degrees Kelvin. This discovery has been challenged recently and might not even exist. Tau Ceti at 11.7 ly, has a few planets which come tantalizingly close to the habitable zone. Beyond that there is nothing interesting (that I am aware of) within reach.
Currently, our best technology today is inadequate to the task of interstellar travel within a reasonable factor of a human lifetime. At stands today, we can propel very small masses to well-past Earth’s escape velocity (11 km/s). With great expenditures in fuel and/or through carefully choreographed flybys of other solar-system bodies, we are readily able to exceed the escape velocity for the solar-system (16.3 km/s). Currently, the fastest man-made object is the New Horizons spacecraft, which is speeding out of the solar system a sun-relative velocity of 45 km/s. At that speed, if it were in fact headed towards the Alpha Centauri system… it still would take 80,000 years. NASA has some long-range designs which aspire to reach upwards of 200 km/s which is 0.067% the speed of light. Which gets you there in just under 6655 years.
While it is physically possible to propel a material object through space at some fraction of the speed of light, the technical challenges and dangers to that object begin to rise exponentially (past a very low threshold).
The most daunting of these technical challenges is that interstellar space, and most certainly interplanetary space, is not empty. There is all sorts of ice, rocks and other stellar leftovers floating between the stars. Consider hitting a 1 kg stationary object at 10% the speed of light. A quick back-of-the-envelope calculation shows that the kinetic energy of the collision is equivalent to 110 kilotons of TNT These kinds of energies are incredibly difficult to absorb or abate if encountered and very difficult to avoid… if not impossible.
Certainly colliding with any sort of macroscopic object will be catastrophic if/when it occurs, however the interaction on the atomic and subatomic levels will be constant and unrelenting. Interstellar space poses all sorts of radiation dangers intrinsically, without regard to motion. However, when an object is traveling at relativistic speeds, these dangers increase exponentially.
Interstellar matter such as gas molecules, energized electrons or other charged or uncharged particles pervade the galaxy… existing almost everywhere at one concentration or another. A constant stream of particles will be bombarding the leeward surfaces of an object traveling at relativistic speeds, imparting tremendous energies to the material as they decelerate or accelerate. What doesn’t penetrate completely will create x-rays, electron-positron pairs and various types of scatter radiation in addition to those particles which penetrate In fact, for these reasons, too little shielding is worse than no shielding at all.
Conventional shielding using dense metals is fraught with problems. For starters, the amounts of mass required to provide minimally adequate protection is extreme. Even the most advanced concepts such as magnetic and/or electrostatic shielding only address charged particles and provide absolutely no protection against nucleonic radiation incurred from neutral particles (a sizable component of the interstellar medium). If there is a solution to the radiation problem, it will almost certainly be both mass and energy intensive and may represent the most intractable obstacle facing interstellar travel.
Considering the dangers from impacts and radiation and the scope of what is required to abate these risks, I suspect that relativistic travel is either impossible or so cost prohibitive, that few if any civilizations have ever been able to justify the effort… and if they have done it… it most certainly wasn’t with biology.. This is not to say that interstellar travel couldn’t be done a slower speeds coupled with multi-generational biology or self-replication technology.
Since the galaxy is very old, this still does not explain why some species, somewhere, hasn’t peppered the galaxy with slow-moving, self-replicating technology… or at the very least, some sort of inert memorabilia, announcing their existence. Given the probabilities of life, over the aeons of time that the galaxy has existed… almost anything that is possible should have already happened. Logically, it seems to follow that we are either very unlucky that nothing has drifted our way, or the first species to have reached this level of sophistication or so rare that we are virtually alone in the galaxy.