Otto Weston wrote:
The Tyranids were created (Either Evolved or designed as a weapon, either way irrelevant) and spread out from their native galaxy. Feeding on anything and everything.

They've spread to multiple galaxies (we can't be sure of the number), but we have evidence of that because the fleets aren't all coming from one source - they're coming from different directions, and therefore different galaxies. There's another piece of evidence that proves they're not coming from the same galaxy; the hive fleets fight each other when they come into contact with one another - which suggests that they grew apart and developed separately, possibly spending thousands of years without contacting other hives.

There is some other fluff that suggest the Tyranids have been to our Galaxy before. In a Ciaphas Cain Novel there is a frozen hive ship that's older than the Imperium. We can disregard this information though, because even if it's canon -- it doesn't really have any bearing on whether the Tyranid Situation now is really bad or not.

The reason why they're coming to the Galaxy appears to be the Astronomicon, the hive fleets are bee-lining straight for it. I think there is a simple explanation as to why; when a Tyranid Hive is done eating a Galaxy, they'd have to set out randomly towards another Galaxy. This has risks. The Galaxy could be empty/dead/eaten by another Hive... therefore the Hive Fleets would want to minimize risk when choosing another Galaxy to eat. The Astronomicon is a giant ringing dinner bell, its existence proves that there is at least something to eat here and so the Tyranids are attracted to our Galaxy. On this premise, only Tyranid Hive Fleets which have finished eating their Galaxy would set out towards ours, there could be many more hive fleets that are still consuming galaxies but have their eyes on ours next.

It seems like the Galaxies around ours have been remarkably empty, considering that the Hive Fleets that have been encountered (each containing a Galaxy's worth of biomass), were defeated so easily (relatively).

As to the extent of the problem, let's tally up what we know:
- Multiple Galaxies worth of biomass has been consumed.
- Multiple Hive Fleets have been attracted to our galaxy.
- Each Hive Fleet has consumed a galaxy's worth of biomass.

The main problem is the amount of biomass the Tyranids have collected in those galaxies. If one of the fleets manages to consume a Galaxy with even half the biomass of ours --- It would be game over. That many organisms working with a singular focus. Totally unstoppable.

So in my opinion, either (Galaxies around us were relatively empty and we've been lucky), or (the tyranids feeding on galaxies with high amounts of biomass aren't done eating yet). If it's the former, we stand a chance of taking on each Hive Fleet as it comes and we might be able to cause the extinction of the Tyranids if they keep on coming in small(ish) clumps. If it's the latter, we're fethed.

kungfujew wrote:
If the above is true, then it is also likely that their home galaxy was much easier to consume than ours is proving to be. Personally, while the I certainly agree that the Tyranids are a galactic menace that have the ability and potential to disrupt the natural progression of our galaxy, our galaxy is more dangerous to them, then they are to it.

kungfujew wrote:
Given the age of the universe, there is a specific finite amount of time the tyranids had to evolve, consume their galaxy and set out to find ours. Also, that time is quite smaller simply because of how long it took for galaxies to form and for stars to have enough matter other than hydrogen surrounding them to form planets. To get a planetary system like ours with heavier atoms in the abundace you find in our solar system you need a third or fourth generation star. Which also takes time. Forming planets also takes time. So the Tyranids likely haven't been at this for as long as one might think.

Considering the timeferame involved in traversing the emptyness between galaxies it is most likely that the tyranids are from one of the smaller galaxies in our local group as they are also closer to us. Those galaxies are mostly dwarf galaxies and contain but a fraction of the mass and number of stars found in the milky way, as the the our galaxy is the most massive galaxy in our local group. Since the tyranids are the natural enemies of every living thing in the milky way and have no allies here, to look at the situation in terms they would understand, from an available biomass perspective they are at an inherrent disadvantage. And that's not even taking into account how much energy is wasted in the vast empty blackness between galaxies, where there is nothing for them to feed on.

 Tyran wrote:

 Hansisaf wrote:

kungfujew wrote:
[...]I've never seen in any of the tyranid codexes an incident where one hive fleet attacks another.[...]

It's in one of the Ciaphas Cain novels. Hormagaunts from different hive fleets are in adjacent cells and once the door is raised, they kill eachother.

There is also Kraken and Leviathan teaming up in the Valedor campaign. Although it seems that while both fleets are part of a greater whole, there are psychic characteristics that differentiate each fleet. It's like if Leviathan used Windows and Kraken used Mac. Such differences can eventually be overcome and the 2 fleets may become one, but it takes time.

Although it seems that the Shield of Baal campaign also has it's own different version of the Tyranid Hive Mind... let's just agree that the 40k fluff is a mess of contradictions.

Iracundus wrote:
Tyranids have bypassed dead Necron tomb worlds that were in the way before, precisely because they offered little yield for the effort involved. A blasted radioactive wasteland with only a few scattered feral survivors should fall similarly into the low yield category.