If all extraterrestrial civilizations in our galaxy were so far away from Earth that they were effectively invisible to us, despite being within the observable universe, it would have several implications and consequences:
Lack of direct contact: The most immediate consequence would be that we would be unable to establish direct contact or communication with these civilizations. The vast distances between us would make it extremely challenging to exchange signals or information in a reasonable timeframe.
Limited knowledge of their existence: Since they are invisible to us, we would have no direct evidence or observations to confirm their presence. We would only be able to infer their existence based on indirect evidence such as astrophysical observations or theoretical calculations.
Limits on interstellar travel: The lack of visible extraterrestrial civilizations nearby would imply that the prospects of interstellar travel within our galaxy might be significantly more challenging than previously thought. It would suggest that either the development of advanced civilizations capable of interstellar travel is exceedingly rare, or that the distances between them are simply too vast for practical travel.
The Great Silence: The absence of observable extraterrestrial civilizations could contribute to what is known as the Fermi Paradox or the "Great Silence." This paradox raises the question of why, given the vast number of potential habitable planets in the universe, we have not yet detected any signs of intelligent life. The scenario you describe would add further weight to this paradox, highlighting the apparent scarcity of visible civilizations.
Focus on local exploration: With limited knowledge of extraterrestrial civilizations, scientific and space exploration efforts may focus more on studying and understanding our own solar system and nearby celestial bodies. This could lead to a greater emphasis on missions within our own cosmic backyard, such as Mars exploration or the search for microbial life on moons like Europa or Enceladus.
It is worth noting that our current understanding of the universe and the possibility of extraterrestrial life is limited by our technology, knowledge, and the vastness of space. As scientific advancements continue and new discoveries are made, our understanding may change, and we may gain further insights into the existence and nature of extraterrestrial civilizations, even if they are currently invisible to us.