Would there be time for large life to evolve? A galactic-size organism is 100000 lightyears across, so can only act as a coherent organism on timescales of at least 100000 years, probably much more.

To evolve immune defense against small life, there would have be many instances of less-fit large life being killed by small life, to generate the selection pressure. Unless the large-life can reproduce implausibly fast and life on earth is a massively slow-evolving outlier (to make this idea work you probably need thousands or millions of galactic civilisations each built on the ruins of the last, in our past), I don't see how this can happen.

Another objection: If small life evolves much more quickly than large life, we'd expect small-life galactic civilisations long before any large-life worthy of the name evolved, and could easily technologically outmatch any evolution of a large-life immune system.

A lot of people upvoted this post (it's at -1 and 47% positive). Why would you do such a thing?

Here I want to highlight a different possibility ... The large ones are, let's say, mostly dark matter, galactic in scale, and stars and planets for them are like biomolecules for us ... And it explains anthropically why you're human-size rather than galactic-size ... Two questions arise - how did large life evolve, and, shouldn't anthropics favor universes which have no large life, just space-colonizing small life?

(The post seems like a clear case of privileging a hypothesis, which is additionally implausible. That Mortimer Q. Snodgrass did it could indeed explain the murder, except in this case Mr. Snodgrass is additionally postulated to be an (ordinary) undead pharaoh; and the questions arise about where was Mr. Snodgrass the night of the murder etc.)

The large ones are, let's say, mostly dark matter, galactic in scale, and stars and planets for them are like biomolecules for us; tiny functional elements which go together to make up the whole.

In what sense do the existing stars and galaxies look in any way designed or organized in structures or in any way differing from straight-line extrapolation from the Big Bang without any invocation of life or selection processes? This is the same question as 'how do cells and proteins look different from rocks?'

And it explains anthropically why you're human-size rather than galactic-size: small life is more numerous than large life

That seems likely to be true - regardless of the merits of the "large-life antibodies" idea.

Last week I had an idea very similar to the "cosmic immune system out to get us" concept expressed above, as a result of reading the book "The Life of the Cosmos" (1997) by physicist Lee Smolin, who paradoxically is an opponent of the anthropic principle, preferring his concept of cosmological natural selection as the explanation of why the laws of physics are as we find them. His is a many-worlds cosmology but the individual universes in the ensemble can reproduce further universes, a process accompanied by the introduction of small mutations into the laws of physics (the genome of a universe) that the progeny inherit, which causes the ensemble to evolve in a Darwinian fashion. (My only quarrel with universe evolution is that there seems to be nothing that can remove less-fit individuals from the scene and recycle their matter for use by the more fit, but we must avoid argument from lack of imagination.) Evolution, when discussed in the idiom of DNA-based genes, is driven by increases in gene frequency reflecting reproductive success. Therefore we predict that the corresponding universal process tries to maximize the fecundity of the typical universe. Take-home: we happen to see a specific physics not because it produces the most observers but because it produces the most universes. (Since "universe" denotes all there is, the word is clearly being misused here. Therefore the units of cosmological selection will be referred to as "compartments" in what follows, and the word "universe" will retain its dictionary meaning.) Abandoning the anthropic principle allows us to view the rise of civilizations of intelligent observers in any compartment as no more than accidental, and probably representing a kind of cosmological cancer. Other suitable metaphors would be "parasitic process" and "failure mode". Examples of these abound, such as feedback noises developing in public address systems, epileptic seizures developing in cerebral cortex and/or hippocampus, or any pathogen-triggered disease in the human body. I am sure that this problem has been regularly impairing compartment reproduction all down the cosmological line of descent leading to us, causing a selection pressure that long ago brought about the evolution of a compartmental immune system, which, of course, is out to get us. But how could little ole Earth threaten the whole observable universe? By someday colonizing the Galaxy in finest Science Fiction style and expropriating the interstellar matter the host compartment needs to reproduce, to make new planets for us to live on. Interesting that the universe thinks we can do it. I guess immune attack can be the sincerest form of praise. The compartmental immune system has in fact probably been taking preventative measures against us all throughout our history, leading to the myth of Old Nick, Father of Lies, The Evil One, etc. It cannot even be dismissed as a myth, it's merely what in science would be called an approximate theory. It appears that the compartmental immune system consists of “mousetraps” for the unwary civilization (perhaps baited with goodies like fossil fuels, the use of which is calculated to Venus the planet in a runaway greenhouse effect that strikes without warning, and uranium, whose dangers do not need repeating), Shminux is obviously all over this one. The traps would be encoded into the laws of physics with diabolical subtlety, and technological civilizations exactly such as ours have always been the target. Based on this line of reasoning, I am calling for the creation of a branch of science that could be called Defensive Cosmology, to help us spot these traps before it is too late. Who's with me? P.S.: lest you think this worldview is overly pessimistic, I'll mention that there is a place for God in it. He is an advanced technological civilization in another compartment that contains ours, that has repurposed our compartment as an experimental test bed for doing experiments on how to defeat their compartmental immune system. Notice that they cannot solve their worst problem before they have solved our version of it. Our interests are aligned. Nor must they give us the disease in order to try curing it, as we do with lab rats. That would be totally sinister. It would also be redundant, because all compartments always naturally come with immune systems. It remains to be explained why solving our problem for us would be any easier than trying to solve theirs directly. Here is what I think could be the reason. The entropy, or information content of a black hole has been shown to be proportional to the area of the event horizon, thought to be a kind of abstract surface. But what if it somehow has a claim to being a literal surface? If black holes are the daughter compartments our compartment has produced, as Smolin suggests, then we can unpack this to mean that these daughter compartments are flatlands! The contents are all on the surface which is locally two dimensional. Everything in these compartments is a total open book to us. We can intervene absolutely anywhere in these worlds, and see absolutely anything in these worlds. Wouldn't you say that that is an experimenter's dream? We are to God as these flatlands are to us. I can reconcile this we-are-a-surface idea with the principle of relativity and the need for a living compartment to have internal motions, if anyone is interested.

I'm currently sleepy, aching and overfed myself a few hours ago, so I'm just going to throw this article here. I'm not in a state to articulate why this is relevant, but I'm dead certain that it is.

If large lifeforms are not nearby, how would we distinguish between their Dyson spheres and the (observed) prevalence of dark-matter?

Not sure why people hate this post so much. Consider that stars (I mean the celestial objects here, not human celebrities), for example, pass most definitions of lifeforms, except those privileging some specific form of metabolism, like carbon-based. Eh, maybe they even pass that one, since many are carbon cycle-based.

Whether Nature has a defense mechanism against large interventions is a testable theory. Some of the consequences have already been confirmed on the small and medium scale. Some examples: Easter Island, antibiotic-resistant bacteria, global warming. We narrowly avoided (by an order of magnitude or so) igniting the atmosphere with thermonuclear explosions. A grey goo might still do us in.