Ah here is that quote from Christopher from a while back:
Referring to the map in Star Trek: Star Charts:
Bold added for emphasis.
The problem -- and those maps illustrate this quite well -- is that human beings have no ability to grasp just how immense the Milky Way is. We have no reference for it. We may have some idea of the difference in size between a city and a continent, or a city and the whole Earth (although most people's estimate of either of those is probably way off), but the difference in scale between a single star system and the entire galaxy is immensely greater.
One way to look at it is in terms of numbers. The galaxy has something like 4-500 billion stars. Even if the Federation could visit a new one every single day, it would take more than a billion years to visit every star in the galaxy. In three centuries at that rate, the most they could cover would be 0.002% of the galaxy. Even in that 4-5% covered in Star Charts, the vast majority of star systems would probably have never been visited by anything but unmanned probes at best.
Even if we conservatively assume that only, say, one star in a million has sentient life, that's still nearly half a million intelligent species, so if you could visit one per day, it would still take over 1,300 years to contact them all. And in Trek, sentient life seems to be far more common than that.
One way to look at it is in terms of numbers. The galaxy has something like 4-500 billion stars. Even if the Federation could visit a new one every single day, it would take more than a billion years to visit every star in the galaxy. In three centuries at that rate, the most they could cover would be 0.002% of the galaxy. Even in that 4-5% covered in Star Charts, the vast majority of star systems would probably have never been visited by anything but unmanned probes at best.
Even if we conservatively assume that only, say, one star in a million has sentient life, that's still nearly half a million intelligent species, so if you could visit one per day, it would still take over 1,300 years to contact them all. And in Trek, sentient life seems to be far more common than that.
That tiny dot contains the entire Federation, Klingon, Romulan, Cardassian, Tholian, Breen, and Ferengi territories with plenty of extra room besides. You can also see how tiny the space is between the Idran system and the Founder homeworld (and frankly I think the book greatly overestimates the scale of things in the Gamma Quadrant), and you can see the very narrow line representing Voyager's journey (and keep in mind that the ship actually skipped over most of that line thanks to various transwarp jumps and wormholes and the like, so it's really just a few short, scattered dashes along that line). All the exploration we've seen in all the Trek shows has covered only a few tiny specks of the galaxy. Even if you throw in the "Approximate Limit of Explored Space" outline around the UFP and its neighbors, I'd say the total explored volume adds up to less than 5% of the entire Milky Way. And that map is based on an old assumption about the galaxy's radius -- we now think the galactic disk might actually be 60% wider, meaning it would have over 2.5 times the total area. Which would mean the explored total would be less than 2% of the whole.