Little excerpt from your manual for you here (I advise you take the time to read it some parts are very vague like allegiance but it provides a useful foundation)-

"Broadcast Center – Your empire can spread its culture to nearby worlds through the
broadcast center, as can be seen through phase lanes taking on an empire‘s color. The
stronger your culture, the faster it will spread and push back enemy culture. As each planet
comes under more of your influence, its allegiance to your empire will increase. When an
empire‘s culture is strong enough, a planet could even revolt and become neutral. You
can‘t colonize a neutral world that has revolved due to an enemy‘s cultural influence until
that influence is reduced. In addition to planetary effects, each race can unlock special
bonuses to its ships stationed in friendly culture."

Now that is how you fix the problem some. Mind you your capital gets 100% allegiance and every jump a planet if from that it gets -10% allegiance. Also enemy culture lowers allegiance as well which is why you need the broadcast centers to fight the "culture war" so to speak.

I think the games tutorials cover this, but I can't remember for sure. They may be worth a fast play through for you. They are very short.

If at all possible you should relocate your homeworld from the edge of the map where you started to the middle of your empire or on a multi star map by a jump to the star.

At the point where your empire nudges up against someone elses, you need more culture centers.

I've had 3 on a planet and still almost lost it, because the next planet along had 5. When I added 3 more, the situation reversed, but I was down to 3% before the situation reversed.

I've had 8 on a planet, using all its civilian slots, and it kept the next planets along from being colonised. But thats rare to be able to build that many.

The other thing you can do is put more culture centers on adjacent planets so the affected planet gets more broadcast at it, strengthening its own.

If the planet being affected is a block point, to keep an enemy at bay while you concentrate on other things somewhere else, then its essential to give it as high a culture as possible, as if the enemy cannot get you militarily, they will build more of their own culture to try and swamp yours.

The moment you see coloured lines on the map, you know enemy culture is active. At that point, you must have researched culture and be actively building the broadcast centers. If one of those coloured lines is touching one of your planets, then its in danger. From that point on, you should be building at least one broadcast center on every planet, and the closer you get to the enemy strongholds, the more culture you build. It may seem a waste to build culture on planets not near an enemy, but it does travel a long way, and it does overlap, so every bit helps.

Or you could not be naive and just implement a raid to destroy their culture centers and any production facilities near the lines of battle.

An enemy fleet isnt gonna stop a raid of 20 lrf before they take out their designated targets. Then again if an enemy fleet is there my 20 illuminators might go on cap killing duty instead.

obviously if you are conducting such a raid you can expect a fleet to show up, which lets you drop a real fleet somewhere else. I didn't expect to have to explain basic diversionary tactics. That is what I get for assuming most humans would already take such things into consideration. Then again I shouldn't be surprised, now should I?

Note to self, start addressing forum goers the same way I would kindergartners.

If you are using raids as diversonary tactics, then fine, I guess you play maps where that is possible.

I still honestly dont see the point, even of using them for diversions. I just hit them hard and wait for them to run. Why bother diverting them when you can make them run ?

I tend to play maps with minimum phase lanes, so there is nowhere to run a diversion to.

Playing kindergarten games btw doesnt win you any freinds, or build any credability.

If your going to advocate particular ways of doing things, then I guess we all need to also add the conditions under which we play, since everyone plays with completely different maps, and one strategy is not necessarily any good on a different map type.

Most of the time the differences like this one are based on comparing apples with oranges. It doesnt make sense until you identify the fruit tree !

Additionally, make sure you have a Capital (HW) planet.  If you lost it, all of your colonies will be 25%, 35% with broadcast centers.